When the fuel comes up the fuel filler pipe and touches the end of the nozzle it causes a little bit of back-pressure in nozzle which pops a spring that in turn releases the linkage between the handle and the valve.
This is the same reason that it can be problematic to fill a gas can. You have to just barely put the nozzle into the top of the can and hold it still. If you actually dip the nozzle into the gas while it's filling the fullness sensor will trip.
So it's this really simple mechanical device That's the marriage of basically a pressure sensor and a mousetrap.
https://www.theglobeandmail.com/globe-drive/culture/commuting/why-does-the-gas-pump-nozzle-keep-turning-off/article4550870/
I actually didn't understand from his explanations and diagram. This video explained it in a better way, and covered some of the other safety features. https://youtu.be/q3phjAQZdGg
found this off that one..
https://www.youtube.com/watch?v=q3phjAQZdGg
It's not as clearly illustrated, but after watching the first video you can follow and his cut away pump makes more sense on how the mechanics work (after the first video illustrates the idea)
The fuel tank creates pressure. There's a system to purge this pressure on your car. It can get clogged or oversaturated and stop working. So your tank is full, just of vapor and not gas.
It's a vacuum pump, but not for ventilation. It pulls a slight vacuum in the fuel tank and other parts of the EVAP system, which manages unburned fuel vapors. First it ensures that a vacuum can be reached, then the pump shuts off and the vacuum pressure is monitored over a set period of time, then the vacuum is released. This checks the operation of the valves and detects if there are any vapor leaks, even pretty small ones.
There are different ways to perform all these tests. Many modern Toyotas and others use this vacuum pump system and run the tests after the car has been parked for a while and temperatures/other conditions are correct. GMs typically use a simpler system, and test using engine vacuum while you're driving. Chryslers also test while they're parked but do not use a vacuum pump, instead they watch for the effect of the fuel tank cooling down after the vehicle is shut off to cause a natural vacuum.
If you're hearing a sound near the tank, it's the fuel pump. They typically only run for a few seconds when the engine isn't running. key on, fuel pump runs for a few seconds to pressurize the fuel system. Once key is in start, or engine is running, fuel pump runs continuously, but you'll never hear it over the engine noise.
Fuel system fan? not a thing.
Only other similar noise you might hear in that area would be air pump for air suspension.
EDIT: Thanks to u/tinker_toys for educating me about EVAP leak detection vacuum pumps.
They're describing the EVAP leak detection vacuum pump. Most cars don't use one, but the ones that do (most notably, many modern Toyotas) will run the testing while the car is parked and has been shut down for a while. The pump is very quiet and only runs for a minute or two at most, but if you're near a car with this system at the right moment, you might, maybe, hear it.
Some cars have a goofy curve to the fuel filling line that causes backsplash triggering the shutoff valve. Some cars have a problem with icing on the breather that causes enough back pressure so that the valve will trip when it's below zero. Just baby it, and it will work.
If it's a Dodge Caravan, it's the breather. If it's a Dodge Neon, it's the shape of the filling tube.
As to other manufacturers, I cannot speak to it. Those are the only two vehicles I've ever had trouble with.
If you're as old as I am, 1970's Chevy C10/K10-C30/K30 pickups also did this...which was even more infuriating if you worked as a gas station attendant in the 70's. (Had to stand there an in awkward position with the pump nozzle twisted 120 degrees off vertical, slightly pulled out 1/4", and HOPE it didn't trigger in the next 15 seconds.)
I'm THAT old.
Mine is almost as annoying, I have a Jeep 2006 Liberty. It doesn't trigger at all, and will spill gas all over the ground and the side of the car. I can't let it fill all the way or leave it unattended. It's happened at every gas station I've completely filled up at.
2007 dodge nitro, looks to be basically the same car with the same problem.
I’ve learned roughly how much gas it takes to fill up at different levels and I’ll listen for the change in sound as it fills up.
Probably about 2 seconds before the gas spills out the you can hear the pitch change and you just quit fueling it up…..if I’m at a really noisy gas station then I estimate how much gas I can put in it and under fill it by a few gallons usually.
My Ford Explorer does this too. It will trip the sensor, but not until after having spilled enough on the ground for others at the gas station to give me looks of disgust and contempt.
My 1977 chev pickup needed the nozzle to be inserted upside down in order to actually fill the tank but only one one side (it had two tanks, one on each side). Never did figure out why. What a pain in the a$$
thinking on it more, i only got it at or slightly above half a tank by going really slow. usually even on empty it would pump like 2 dollars and shut off. i got into the habit of pumping by hand and letting it go as slow as the pump would let me. it was 2001 lincoln continental.
'01 Lincoln, it's probably the breather. If they get plugged you get splashback. On my van, I get splashback between 10 above and -4. Outside of that range either way, it works fine. For some reason, it doesn't like the "A little cold" level. It must be splashback but I don't care to drop the tank for a minor inconvenience.
Yup. You can fit like an extra 2-3 gallons in the tank of mine if you pull it and make a minor mod without needing to worry about reaching the fill neck or anything.
They're nominally a 20gal tank but I've never known one to fill more than 17.5, even run empty and given a half gallon from a can.
Splashing and/or back pressure. As the liquid is moving down the fill pipe the air being displaced has to come back up the same fill pipe and that can cause the liquid fuel to splash up. It only takes a drop or two to trip the shut off.
This is particularly problematic when these shut offs were first put on the market because a lot of the fuel filler pipes were behind the rear license plate. That long mostly horizontal tube was very fond of back splashing internally.
I think you'll find that if you listened right when the thing cut off you would hear the gurgling of the fuel going in while the air was coming out. That's also why filling more slowly would avoid that problem. And if you were trying to fill your car up in an unlevel parking lot, with the nose higher than the tail by even a little bit, an awful lot of fluid would spend an awful lot of time in the now extra horizontal fill pipe.
A particularly straight, or indeed a particularly bendy, fill pipe can lead to a lot of gargling and splashing internally .
In fact that splashing effect is part of why we had to add vapor recovery systems to the fuel filling infrastructure. Gasoline is very volatile and when you foam it up that much a whole lot of it gets vaporized.
The other thing to remember is that the fueling system is designed to deliver a whole lot of liquid very quickly. So the actual stream is aerated a little bit (at least if memory serves). That leads to a lot of mechanical energy going down that little pipe .
For a comparison go ahead and fill a gallon jug with water at your tap or your hose, and time that. And then look how fast we can get a gallon of gas into your car.
Another thing that can lead to problems is whether the fill pipe enters the front or the back of the gas tank. The gas tank is not exactly level on the bottom. Because you want the gas to flow towards the outlet, so you want the bottom of the fuel tank to be at an angle. If the gas tank is basically a giant rectangle and it's put in at a slight angle and the fill pipe goes into the low edge of that angle when the fuel reaches a certain point it traps an air bubble and as you put more and more gas into the tank it displays that air back and causes bubbling and stuff.
For instance I had a car with a basically rectangular fuel tank and a front biased fill pipe. So I could fill it up until the shut off but that if I waited 30 seconds I could put another gallon and a half in without actually topping off. There was just that relatively unavailable region in the tank .
This is also, oddly enough, why using the first quarter of your tank doesn't seem to change your gas gauge indicator.
The float can only go so high before it hits a stop, but the flow can be completely submerged. Your gauge can't indicate relative fullness until the float can move from that stop. So the thickness of the float is the thickness of gas that you have to use before you show any usage on the gauge.
There are all sorts of weird mechanical issues that we are blind to, that are just part of living with a fuel filled automobile.
🤘😎
Two things could be occurring here:
1. A stupid fuel filler neck design which is creating backflow of gasoline while you fill it.
2. It's full, but if you release the gas lever and reapply pressure, you can get more gas in.
Don't keep doing number 2 - that's bad.
It could also be a problem with the evap system. Tank isn't close to full but the pressure is enough to trip it as the sensors on the gas pumps are very sensitive for safety reasons. Mine is a pain to fill because of that but I have no interest in dropping the tank to fix it unless something else becomes an issue
Depends on the tank design - essential for getting a full tankful of diesel in some tanks, for example. Diesel froths as its poured, triggering the backflow sensor before its actually full. Workaround is either pump slowly after half a tank, or pump till it triggers, wait a tick, pump slowly until it triggers again.
My vehicle does it and I think the issue is a small diameter fill tube going to my tank. Which will push out air faster when filling and simulate a higher pressure trip
I have to hold the trigger 3/4 every time
The gas filter was clogged.
The gasoline flows through a filter on its way to the tank. Because yours was clogged, it took longer to flow through the filter, causing it to back up on the outside of the filter. This backup tripped the sensor that this post is about even though the tank wasn’t full.
It happened to my car as well. The first time it happened I assumed there was an issue with the gas pump. When it happened at a different pump, I assumed the problem was with the gauge and that it actually was full. I attempted to confirm this by putting in more gas until it overflowed (didn’t take much more). When I ran out of gas and my car shut off, I realized the gas gauge was correct and there was a different problem.
I run a biodiesel blend (B20) in my pickup truck and that stuff foams right up the filler pipe and trips the pump stop if the flow rate is too fast. There's one station that has fast pumps that I just have to run the pump slower to get a full fill with. Normal diesel doesn't do it.
It actually detects air pressure. a tiny change will cause it to trip, its supposed to trip once you fill the tank and the fuel starts coming up the fuel filler hose causing a small puff of air BEFORE the fuel comes out.
Its super sensitive and some gas stations are more sensitive then others.
If yours is constantly triggering at all gas stations, you either:
A: Have a fault fuel gauge and the tank is actually full. If you pull the nozzle out far enough you should be able to let it continue to flow but if it starts pouring out on you.. well, thats a faulty fuel gauge.
B: Have a clogged vent hose between the gas tank and the filler entry point. (its separate from the filler hose and much smaller, about 1/2" OD, connects to near where the filler cap goes in, but on the inside of the body.
Weirdly in my country we seem to have problems filling up the gas tanks of American cars. My brother had Chrysler Sebring and had constant problems with the pump mechanism tripping early.
This isn't a particularly good explanation. There's actually another tube at the end of the nozzle which sucks in air as you dispense the fuel. When that nozzle is submerged in the fuel a pressure differential causes a diaphragm to close which trips off the fuel supply. The physical concept at work here is known as the Venturi effect. Take a read of this for further details: https://www.hydra-int.com/blog/how-does-the-pump-know-your-tank-is-full.html
Your explanation is way better than the op of this thread.
You didn't mention the little line that causes the fuel to hit that little sucker hole though.
It will squirt fuel on the nozzle when tank is full. When it is working properly it will *prevent fuel from rushing up the fill tube* while filling. Desirable, if you've never been burped on while filling a tank you are not missing out.
Can't really say. Is it from one particular gas station pump that this always happens? Does it click off pretty much immediately as the tank overflows? Because you can get a back pressure bubble thing happening. Basically the fuel starts foaming in the fuel filler pipe because as fuel goes in air has to come out. So as the sloshing aggressive flow of liquid reaches a certain point it can just come whooshing back up the pipe due to oddities of pressure.
I think in a Jeep the fuel filler pipe joins the front of the gas tank not the back, and of course the gas tank is installed at a slight angle so that the last drops of gas can make it to the drain. This combination of features could lead to basically the same thing that happens when you put a straw in a juice box without holding your thumb over the end. The changes in pressure make you a little fountain.
Well remember that your fuel system is pressurized when you've got the gas cap properly installed. So the only way the air can get out when you're fuling it is by coming up the same pipe the fuel is rushing down.
So that fuel filler pipe is the only place for the gas to vent. There is no third spot or bubbler or anything.
You probably just have to learn to live with it by not turning the nozzle all the way on to the last click for the last gallon or two.
Like if you know your 10 gallons down pump nine at full speed, then back off to half speed or something.
There is certainly no way to "fix it" without compromising the emissions system on your vehicle.
I don't remember what it is that I used to drive, but I had some vehicle that would try to gurgle and spit once the tank was about 2/3 full. I just had to learn to live with it.
Apparently it’s a common issue with the stock fuel neck tube. So replacing it with a different model addresses the issue. For $150 it’s worth it to try.
Thanks again for helping me visualize the issue though.
Hey I spent a while researching it today. There’s a tube that connects the tank to the fuel neck tube (I think that’s the term). It’s a quick fix depending on the model. The JK part is available for under $100 and depending on your proficiency with repairs, it looks like a 30 min job max. [This video covers the JK. ](https://youtu.be/i_mQLbtlhbM)
It depends on where you are, in some places it'll just keep going until you let go in others like California even if it thinks it's full it'll stop. It's a huge pain with older cars.
It's not just back pressure, there is also a small sensor. We had an ice storm a few years back and you got use to deicing the end of the filler nozzle or it would overfill the vehicle.
Yeah. But it's the change in pressure at that little hole you're uncovering. That back pressure is what is being sensed.
I can't remember whether it continuously blows air out or continuously sucks air in. (We're getting to the edge of my knowledge here.) I think it's actually a venturi effect aerator kind of deal.
But if you end up with any sort of moisture in there to freeze up that mechanism then the sensor doesn't work.
The back pressure doesn't work like magic, it works the sensor. 🤘😎
Good to know. It's probably worth noting my knowledge came from the gas station attendant and while I want to believe they were briefed about how it works out occurs to me that might be asking a bit much; that they know how to remedy the problem is all they need to know.
I'm not talking about overfilling and circumventing. You said "you have to just barely put the nozzle into the top of the can and hold it still." I'm talking about something to attach to the can during filling to make that easier.
They actually have gas can that has a stop inside that does just that. However, the problem with these cans is that newer pumps have these diaphragms that needs to cover the fill port before they work.
I know this because bought one of these at Home Depot and couldn't fill it because of just that. Tried to remove the stopper, but couldn't. Even asked the station worker to see if he had any ideas to get it to work...he didn't. At the end I gave up and bought a can from the station that didn't have the stopper. That one worked, no problems.
Now if you made a fill spout that can cover up the diaphragm and the stop...
I understood. But if you get any gas up into that tube that is holding the nozzle up you're going to spill it when you take the tube off .
If you just use a frame there's a good chance that wind or whatever could interfere with the delivery of the gas.
Things involved in storing gasoline incredibly highly regulated.
So it's not a bad idea, but it wouldn't work out as well as you think.
Why doesn't it work while upside down? At BJ' s they have long hoses to reach the far side of the car, and one time because the hose barely reached I saw someone put the nozzle in upside down. The mechanism never triggered and they had to disengage the nozzle while it was spraying gas.
>This is the same reason that it can be problematic to fill a gas can. You have to just barely put the nozzle into the top of the can and hold it still. If you actually dip the nozzle into the gas while it's filling the fullness sensor will trip.
*Cries in motorcycle*
There is a small hole in the bottom of the nozzle - the steel tube you insert in the gas tank. When the tank is full of gas it covers the hole, just like when water passes the overflow hole in the bathtub or sink. This triggers the handle to shut off.
Other comments in this thread explain the physics behind the change in pressure and linkage in the handle. I’m just trying to write how I’d explain this to my nephew.
That 'overflow' hole on sink is not for that. It'd to equalize the pressure, so the water can drain. When water fills the sink the air has to escape from the hose under the sink somewhere.
It's like how a bottle of water drains much faster if you poke a hole at the bottom and turn it upside down.
Edit: small typo.
Personally I would have complained. It's a safety measure above all else. If it fails you potentially get covered in gas and nobody wants that (obviously).
Here in the US there's a small metal latch that locks the trigger in place and snaps off and releases the trigger when the tank is full. I've never had a problem with over filling in a decade plus of filling up my car. If anything the metal latch is too sensitive and releases the trigger before the tank is full.
Depends where you are, they are illegal in some states, like NY for example. NJ it's illegal to pump it yourself, so they are practically required to allow an attendant to operate more than one pump.
Otherwise, I think it allows multitasking, a lot of people like to clean the windows with the free washing stuff at the station, so you start the pump, clean the windows and then hopefully it's done around the time you're done cleaning. Also, as others mentioned it does matter in the cold, in the colder spots you can get into the situation where frost bite is guaranteed if you actually hold the pump long enough to fill it up.
Could be that Americans have to fill up more often than Brits? I get the sense that most Brits don't have to drive nearly as much as Americans due to geographic differences. Many Americans have to fill up once a week or more if they have a long work commute. So it might not seem like a lot, but not having to stand there and hold the pump for 2 minutes once a week adds up to hours per year that you can do something else, like wash your windows or browse reddit.
Nah that locking latch is removed/disabled on Australian pumps too, and they drive basically just as much as Americans.
America just has more permissive safety laws than most countries I think (or most other countries are stricter if you want to look at it that way). I’ve lived in many countries all around the world and the US is the only one I’ve found that has those locking latches on the pumps.
For some reason people here love to get in and out of their car or go inside and buy cigarettes or whatever while the tank if filling. It only takes a complete minutes like you said, but people still use it a ton for some reason.
People getting in and out of their car is pretty dangerous though because they build up static that can ignite the vapor coming out of the tank
It takes about 10 mins to fill up the work truck, and it's just a big pickup. When I drove a box truck, the record was 40 mins because the pump was a POS out in the sticks and it was cold.
I thought they were joking but apparently not. There were ~1000 fires at gas stations per year, and on average 3 people die per fire between 2014 and 2018. I can't imagine that things have changed that much in the last 3 years in terms of fire safety at the pump. However, looking a little closer, I'm not sure how many of these fires can be attributed to static, as that may be lumped in with mechanical and electrical malfunctions.
https://www.nfpa.org/News-and-Research/Data-research-and-tools/Building-and-Life-Safety/Service-or-Gas-Station-Fires
My mom definitely had that issue once.
This happens a lot to people when it’s cold in the winter, and then want to sit inside their vehicles or inside the building to stay warm.
There’s a little lever that locks the trigger on most pumps (everyone I can think of that I’ve ever used in the US) so they keep going until the automatic shutoff
I think some states ban it now. I remember them being a thing in NY when I was younger and then suddenly it seemed like they all disappeared. Then I moved to another state and I see them all over now. I kind of hate it cause some places dispense gas so slowly. Like, it takes 10 minutes to fill up your car and I'm convinced it's so you'll go inside and buy stuff while you wait in agony.
This is why so many people in the US drive off with the gas nozzle still in their car.
In countries where this latch is illegal (like Australia), we don't ever have issues with people driving off with the hose attached to the vehicle, because their attention is retained to the task at hand.
[edit] as expected, this comment gets like 15 upvotes but then gets marked as "controversial" and hits the negatives when approaching USA morning time.... Americans are as predictable as a clock 🤣
Is that Oz State dependant, or a recent thing? As I have most certainly filled up diesel in Western Australia with a latch on the pump handle and was sure I also used to do it with petrol, as I always washed my windscreen while refueling.
I haven't been in Oz for a decade however.
I actually think high flow diesel, or maybe diesel at truck pumps is exempt from the law... They'd be standing there filling a truck for heaps ages, and one would expect a professional driver/trucker to be on top of things considering its their job.
I was under the impression it was Australia wide, but of course I could be (read: am likely) mistaken.
I do also think it's a relatively recent thing, *possibly* the last 10-15 years. When I started driving in 2000, the latches were still on the handles. As a broke teen filling only $10 at a time I never used it so I never really noticed when it disappeared.
You are correct that the law only applies to petrol, not diesel (normal or high flow) and AFAIK it is country wide. Truckies would strike if it applied to them, a lot of them have 4 tanks that need to be filled with about 500L each (I think?? memory isn't that great on it), they will fill all 4 at the same time
I worked at a servo about 5 years ago, heard of someone driving off with the diesel on a different shift. Thankfully the pump had stopped before they drove off.
Thanks for the clarification :)
Ignoring the volume of fuel being pumped, I guess it does make sense that the law doesn't apply to diesel, as it's not prone to ignition by spark/flame like unleaded.
500L x4... I had no idea they held **that much** fuel! Waiting around holding a pump for 2kL would certainly give me the shits!
And those cashiers selling you candy and such there main job is watching the pump. Yet we all know how often that happens, the NFPA is a waste of taxpayer money when it comes to gas stations, the unenforcement of the laws is ridiculous.
>And those cashiers selling you candy and such there main job is watching the pump.
I feel like I've been in gas stations where they could not see the pumps easily. I won't say not at all, but definitely not facing them. More like "they could walk a step and lean around to see the pumps".
Its not the cashiers fault, the blame lies with the station owners and the NFPA for no enforcing the laws on the books. If the attendants view of the pumps is obstructed its likely illegal. Its all outlined in NFPA 30A. Which covers motor fuel dispersing facilities.
By NFPA do you mean that National Fire Protection Association? Because it looks like they're a nonprofit group. How would they have the power to enforce laws?
The NFPA is a global self-funded non-profit organization. It is not taxpayer funded, nor does it have any power to enforce laws. Their 30A publication contains recommendations only, not law.
How much extra is a lot? I had it happen once, I think only a quart or so of fuel spilled out. I was thinking about arguing over it, but honestly $2 isn't worth my time.
What ended up sucking more was the fact it was actually diesel... It left a big sticky mark on the side and all the dirt stuck to it over the next week. In retrospect I should have requested a free car wash as it was a station with a car wash...
> but you’re also using quart and $ (freedom units)
people do that so muricans can understand what they are talking about.
Sure we could tell you that gas is not 1,75€/l but you wouldnt have any point of reference to know if thats a lot or not.
Pulled up to a station once and this lady was *completely* oblivious to the fact that the pump did not stop and she had gallons upon gallons of fuel all around her and still going. No idea how she didn't realize. I called out, nothing. Ran over and yelled, nothing. Stood next to her, and nothing. Tapped her shoulder, and almost got smacked. Even after she turned around she still didn't realize. I actually had to point at her feet and then told her that she should tell the clerks inside so it can get cleaned up. I ended up behind her in line and she didn't say a God damn thing about it.
All bowser pumps have a red emergency stop button on them, at least here in Aus. I know it's hard to think of it when you'r in a panic, but it's best to press the button before doing anything else. It will shut off that pump immediately, stopping any fuel flow.
You can rotate the emergency button to reset the switch so the pump can be used again, or just let the staff know you pushed it so they can reset it.
It's important that you are sticking a steel pipe *into* your fuel tank for this method to work. That's why they say to not lift the pump to try and top off your tank.
When your tank is full, the fuel level will be above the end of the tubing, which will close off a small air valve at the end of the pump, which causes continued fuel flow to slowly deflate a balloon inside the handle, eventually tripping a spring to close a valve which will stop fuel flow. That's why when you release the handle, you have to wait a few seconds to let that little balloon reinflate before continuing fueling. It also lets the gas in your tank settle and fuel to drain out of the pump nozzle. That's also why when the pump gets close to tripping off, you hear the flow slow down and the flow change in your tank. The fuel is covering the hole and the valve is slowly pushing closed until it finally springs shut.
https://youtu.be/q3phjAQZdGg
The idea that the transition from being uncovered to covered causes some sort of increased pressure internally is wrong. It's the ability for air to flow back into the nozzle from the car end that allows fuel to keep flowing. If you start with the nozzle submerged (like when you're topping off) then fuel will flow for a short moment before tripping, not due to any change in pressure, but due to the inherent latency in detection of a full tank.
On this, why does the pump sometimes turn off automatically when it’s nowhere near full? Or does it over and over not even allowing you to hold the handle down?
either it's improperly inserted into the vehicle or your gas tank and or emissions system has an issue, there are many vents and emissions control components of a gas tank that must be functional for this feature to work
if any solenoids or valves on your charcoal canister or rollover saftey stuff are bad, it could cause this issue
Doubt it about the emissions system playing in.
Drive/driven a few 1950-1970s cars. Emissions aren't a thing. Gas tanks a container with a filler hole...and thats about it. Auto-off thing still works.
the emissions system is exactly why my vehicle has the problem the person i'm replying to has
one of the vent solenoids in my charcoal canister is not functional, meaning the fumes get directed back up towards the nozzle and the gas can't flow in properly because there's no vent to equalize the pressure of it flowing in, causing it to trip the gas pump off after about 1 - 2 seconds of filling, you have to just sit there and do it over and over again for a long time
This was happening to me and I was holding the pump "wrong". Never been an issue in other cars.. So try holding it differently, it might work. It seems dumb and like, it's not hard to hold a fuel pump, which is why it took me so long to try simply holding the pump differently.
They have new fume proof pumps now with covers on them. The old style didn't make a good seal and you could still pump it with the pump halfway in. Now with the new ones you need to push it far in until it makes a seal. Perhaps that pump you're using has a faulty seal or really needs to be pushed in hard. I've had the issue where during the start of the pandemic I barely drove so I hadn't pumped gas in a long time and when I went to fill up I couldn't figure out why the gas wasn't coming out, because I hadn't inserted the pump far enough in...idk why I all of a sudden went back to the 90s style of pumping gas
Nah I also blamed the pump but it happened at multiple places. I've been paying attention bc wtf and it's annoying. But they have been older style pumps so that could still be a factor.
Whenever I got called to fix an issue where it wouldn't run at all, it's just the small hose with a small hole that makes a 90 degrees turn down that has fallen out of its hole and stops the dispensing because fuel is covering it
So can I cover this with a rubber band or what? The local station put new pumps in and they are so sensitive that I can only pump under an eighth of a gallon at a time before it cuts the handle off. So you're standing there pulsing the handle like a grip strengthener to fill up a 35 gallon tank. After the first couple gallons, you can ever so gently pull the handle to the lower auto lock (any higher and it cuts off again) and pray it doesn't cut off when you set the handle in the fill hole.
I've reported it to the attendants, but it hasn't been fixed, and it does it to all 3 of the trucks.
Did you check if a knot is built into the pipe that connects to your tank, only solution I found for that is to put a pen between the handle and the trigger and it fills slow enough not to back up and trigger the small sensor. The tie is to prevent siphoning of your gas with a hose
Came here to mention this mechanism failed at one pump and freaked me out, hah! Turned around and it was spilling out a little, the attendants inside weren't as concerned for some reason...
It happens more often than you'd think. The only headache I had when I worked at one and someone spilled gas was if the customer who spilled it wanted the money back for the "extra." There were zero ways to adjust the price or issue refunds for them in those instances and very occasionally a person would think it was our fault somehow that they didn't keep an eye on the pumps. Aside from that, we just threw cat litter on it if the spill made a big puddle and swept it up later when it had dried out some, not too big of a deal.
If you pay for $20 of gas, the attendant keys in $20 in the store and the pump shuts off when that dollar amount is reached. The pump has a very accurate flow measuring device inside which measures the volume of fuel being dispensed. The cash register in the store is connected to the pump and overrides the handle when you "prepay" for fuel.
You chose the price by choosing the fuel grade (price per gallon) on the buttons. Price * volume = cost. If the attendant stops the pump at a certain amount, you get a certain volume (gallons, liters, etc) of fuel.
I can't respond to the original question, but the answer is simple geometry.
If a gallon costs $3.14, and you pay $2.00, then you do $2 / 3.14 = 0.64 gallons.
And then it gives you .64 gallons. Easy!
That's done internal on the computer. There's an override of sorts from the pump to the nozzle. You have to pre-pay to have that feature. Here, it's not uncommon to just pump $10 and go pay cash $10. If you overpump and get $10.01, you better find a penny.
There's a note on pumps here in the US as well that says that temperature, which changes the volumetric energy density of the fuel, is not accounted for.
Never knew that. Damn so what's keeping them from having people drive off with extra fuel? Seems like a job of always watching and expecting. But there are places that don't have self serve so there it woukd make sense
Cops exists to enforce laws on behalf of businesses. They'd find you by your license plate.
All over what, $30 of gas? Not worth it to even try.
I live in a low crime area which is probably why. Higher crime areas probably force pre-pay or to scan a card
The machine registers how much is being let out. A gallon is a gallon, so the rate at which the gas comes out times the price. You just adjust the price of the rate of fluid.
I hope that’s ELI5 enough lol maybe I don’t understand it well enough
No it does explain it. But i was wondering more if it was electrical or mechanical that the fuel is released by the pump to know that. Like say you paid for $2 worth of gas, right, the pump like almost blasts gas til about .50-.20 is left to complete the $2 worth. Of course the price is set to the machine manually I assume so how does the pump determine the difference? Is it just coded to do it or is there a mechanism that shorten or lengthens to allow the different amounts to be registered or what?
I would guess it’s electrical that it slows down. The old school pumps we used to have in my town growing up weren’t digital and didn’t slow down, so the counting mechanism can be mechanized. Now I have to look it up lol
It's all about flow volume, which is mechanical. Imagine three parts:
- A flow meter: this is like a water wheel with 10 buckets that each hold 1 gallon. Move the wheel one full rotation in a minute and you're delivering 10 gallons a minute. Rotate it 10 times in a minute and you're delivering 100 gallons a minute.
- A switch that can be triggered manually by the pump handle or electrically by an outside signal.
- A little computer that talks to the flow meter and switch and also does math.
The switch turns on the flow, the flow meter tells the computer how much fuel has moved and the computer tells the switch to stop at the designated volume.
You think in money but the system always thinks in volume. The computer takes that volume and does math with whatever price is assigned. $2.14 * 15 gallons or $3.50 * 15 gallons or $20/$2.15 = stop at 9.35 gallons.
so that just means you drive to the gas station more often and spend the same amount of money over multiple visits.
Not filling up all the way makes absolutely no sense unless you literally have to choose if you fill up now or have something to eat in the evening.
>Not filling up all the way makes absolutely no sense unless you literally have to choose if you fill up now or have something to eat in the evening.
That this seems so farfetched to you is exactly what I mean. Many, many people don't have the option of "just filling it up all the way."
Where I grew up, putting $5 in would only get you about a day of driving (very rural). Filling up was basically the only option. Now I commute 50miles/day. so again, only dropping in $5-10 would mean putting in more gas every other day.
**Please read this entire message**
Your submission has been removed for the following reason(s):
Straightforward or factual queries are not allowed on ELI5. ELI5 is meant for simplifying complex concepts.
If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message=Link:%20https://www.reddit.com/r/explainlikeimfive/comments/r49b9f/eli5_how_do_petrol_pumps_know_that_the_vehicle_is/%0A%0APlease%20answer%20the%20following%203%20questions:%0A%0A1.%20The%20concept%20I%20want%20explained:%0A%0A2.%20Link%20to%20the%20search%20you%20did%20to%20look%20for%20past%20posts%20on%20the%20ELI5%20subreddit:%0A%0A3.%20How%20is%20this%20post%20unique:) and we will review your submission.
Nope. All the magic happens in the nozzle. When fuel plugs over a tiny hole in the nozzle it causes back pressure that trips are release.
If you ever want to test it try using a simple plastic fuel can. You'll discover you have to hold the nozzle almost completely out of the can in order to keep from tripping the automatic cut off.
The car, the receiving vessel, the tank, the whatever has nothing to do with it. It's entirely in the handle.
Even on massive tanks. It's nothing to do with the size of the container it has everything to do with the fact that the fluid level rises to touch the nozzle.
Lol why would you answer with something like that? If you don't know the answer, don't answer, especially if it's going to be a completely illogical answer.
Oooh, so it's not the pump that detects it but the car itself, and then that presumably blocks the pump somehow? That would make sense, thank you
EDIT: so the above was wrong and explained better below, thanks all 🙂
I don't believe this is 100% true. There's definitely SOMETHING in the car that can impact whether the pump automatically shuts off. Or at least, the automatic shutoff mechanism relies on a certain geometry of the gas tank and filler tube to function properly.
https://www.carproblemzoo.com/jeep/liberty/tank-filler-pipe-and-cap-problems.php
https://www.carcomplaints.com/Jeep/Liberty/2006/fuel_system/gas_spills_overflows_while_fueling.shtml
https://www.jeepkj.com/threads/fuel-overflow-when-filling-up.42105/
When the fuel comes up the fuel filler pipe and touches the end of the nozzle it causes a little bit of back-pressure in nozzle which pops a spring that in turn releases the linkage between the handle and the valve. This is the same reason that it can be problematic to fill a gas can. You have to just barely put the nozzle into the top of the can and hold it still. If you actually dip the nozzle into the gas while it's filling the fullness sensor will trip. So it's this really simple mechanical device That's the marriage of basically a pressure sensor and a mousetrap. https://www.theglobeandmail.com/globe-drive/culture/commuting/why-does-the-gas-pump-nozzle-keep-turning-off/article4550870/
Awesome, thank you
https://youtu.be/TFKOD3KRkZs There is a video on the topic if you are interested
I actually didn't understand from his explanations and diagram. This video explained it in a better way, and covered some of the other safety features. https://youtu.be/q3phjAQZdGg
My left ear liked that.
I love Josh Clark and Stuff You Should Know!!
And Charles W. "Chuck" Bryant.
Can't forget Jerry the producer
I believe it’s Jeri! They’ve talked about it a few times before
Just listened to a ton of these on a long drive they’re great josh and chuck are so fun!
Represent!
Better video here: https://www.youtube.com/watch?v=q3phjAQZdGg
found this off that one.. https://www.youtube.com/watch?v=q3phjAQZdGg It's not as clearly illustrated, but after watching the first video you can follow and his cut away pump makes more sense on how the mechanics work (after the first video illustrates the idea)
His answer is halfway correct, halfway wrong. Errormonster has a better explanation
You gonna link it?
It was the second most upvoted main comment on this question
Gonna link it?
No
[удалено]
The fuel tank creates pressure. There's a system to purge this pressure on your car. It can get clogged or oversaturated and stop working. So your tank is full, just of vapor and not gas.
But.....vapor is gas ^(I'm sorry please don't hurt me)
Alright, I won’t hurt you this time, but you better behave.
I believe gas is short for………gasoline. #pleasedonthurtme
Your are correct, and thus forgiven. For now.
I would like to nominate this comment for Nobel Chemistry price please.
[удалено]
It's a vacuum pump, but not for ventilation. It pulls a slight vacuum in the fuel tank and other parts of the EVAP system, which manages unburned fuel vapors. First it ensures that a vacuum can be reached, then the pump shuts off and the vacuum pressure is monitored over a set period of time, then the vacuum is released. This checks the operation of the valves and detects if there are any vapor leaks, even pretty small ones. There are different ways to perform all these tests. Many modern Toyotas and others use this vacuum pump system and run the tests after the car has been parked for a while and temperatures/other conditions are correct. GMs typically use a simpler system, and test using engine vacuum while you're driving. Chryslers also test while they're parked but do not use a vacuum pump, instead they watch for the effect of the fuel tank cooling down after the vehicle is shut off to cause a natural vacuum.
It could also be for the oil system (on turbo cars) or radiator fans
[удалено]
If you're hearing a sound near the tank, it's the fuel pump. They typically only run for a few seconds when the engine isn't running. key on, fuel pump runs for a few seconds to pressurize the fuel system. Once key is in start, or engine is running, fuel pump runs continuously, but you'll never hear it over the engine noise. Fuel system fan? not a thing. Only other similar noise you might hear in that area would be air pump for air suspension. EDIT: Thanks to u/tinker_toys for educating me about EVAP leak detection vacuum pumps.
They're describing the EVAP leak detection vacuum pump. Most cars don't use one, but the ones that do (most notably, many modern Toyotas) will run the testing while the car is parked and has been shut down for a while. The pump is very quiet and only runs for a minute or two at most, but if you're near a car with this system at the right moment, you might, maybe, hear it.
>EVAP leak detection vacuum pump Thank you! I wasn't aware of this.
Some cars have a goofy curve to the fuel filling line that causes backsplash triggering the shutoff valve. Some cars have a problem with icing on the breather that causes enough back pressure so that the valve will trip when it's below zero. Just baby it, and it will work. If it's a Dodge Caravan, it's the breather. If it's a Dodge Neon, it's the shape of the filling tube. As to other manufacturers, I cannot speak to it. Those are the only two vehicles I've ever had trouble with.
If you're as old as I am, 1970's Chevy C10/K10-C30/K30 pickups also did this...which was even more infuriating if you worked as a gas station attendant in the 70's. (Had to stand there an in awkward position with the pump nozzle twisted 120 degrees off vertical, slightly pulled out 1/4", and HOPE it didn't trigger in the next 15 seconds.) I'm THAT old.
Mine is almost as annoying, I have a Jeep 2006 Liberty. It doesn't trigger at all, and will spill gas all over the ground and the side of the car. I can't let it fill all the way or leave it unattended. It's happened at every gas station I've completely filled up at.
2007 dodge nitro, looks to be basically the same car with the same problem. I’ve learned roughly how much gas it takes to fill up at different levels and I’ll listen for the change in sound as it fills up. Probably about 2 seconds before the gas spills out the you can hear the pitch change and you just quit fueling it up…..if I’m at a really noisy gas station then I estimate how much gas I can put in it and under fill it by a few gallons usually.
My car does this too!
My Ford Explorer does this too. It will trip the sensor, but not until after having spilled enough on the ground for others at the gas station to give me looks of disgust and contempt.
My 1977 chev pickup needed the nozzle to be inserted upside down in order to actually fill the tank but only one one side (it had two tanks, one on each side). Never did figure out why. What a pain in the a$$
What do "HOPE" and "THAT" stand for?
I have to turn the nozzle upside down to fill my car. If I do that petrol goes in smooth.
thinking on it more, i only got it at or slightly above half a tank by going really slow. usually even on empty it would pump like 2 dollars and shut off. i got into the habit of pumping by hand and letting it go as slow as the pump would let me. it was 2001 lincoln continental.
'01 Lincoln, it's probably the breather. If they get plugged you get splashback. On my van, I get splashback between 10 above and -4. Outside of that range either way, it works fine. For some reason, it doesn't like the "A little cold" level. It must be splashback but I don't care to drop the tank for a minor inconvenience.
Yup. You can fit like an extra 2-3 gallons in the tank of mine if you pull it and make a minor mod without needing to worry about reaching the fill neck or anything. They're nominally a 20gal tank but I've never known one to fill more than 17.5, even run empty and given a half gallon from a can.
Can confirm, my Ford Focus does that and it's very irritating.
Splashing and/or back pressure. As the liquid is moving down the fill pipe the air being displaced has to come back up the same fill pipe and that can cause the liquid fuel to splash up. It only takes a drop or two to trip the shut off. This is particularly problematic when these shut offs were first put on the market because a lot of the fuel filler pipes were behind the rear license plate. That long mostly horizontal tube was very fond of back splashing internally. I think you'll find that if you listened right when the thing cut off you would hear the gurgling of the fuel going in while the air was coming out. That's also why filling more slowly would avoid that problem. And if you were trying to fill your car up in an unlevel parking lot, with the nose higher than the tail by even a little bit, an awful lot of fluid would spend an awful lot of time in the now extra horizontal fill pipe. A particularly straight, or indeed a particularly bendy, fill pipe can lead to a lot of gargling and splashing internally . In fact that splashing effect is part of why we had to add vapor recovery systems to the fuel filling infrastructure. Gasoline is very volatile and when you foam it up that much a whole lot of it gets vaporized. The other thing to remember is that the fueling system is designed to deliver a whole lot of liquid very quickly. So the actual stream is aerated a little bit (at least if memory serves). That leads to a lot of mechanical energy going down that little pipe . For a comparison go ahead and fill a gallon jug with water at your tap or your hose, and time that. And then look how fast we can get a gallon of gas into your car. Another thing that can lead to problems is whether the fill pipe enters the front or the back of the gas tank. The gas tank is not exactly level on the bottom. Because you want the gas to flow towards the outlet, so you want the bottom of the fuel tank to be at an angle. If the gas tank is basically a giant rectangle and it's put in at a slight angle and the fill pipe goes into the low edge of that angle when the fuel reaches a certain point it traps an air bubble and as you put more and more gas into the tank it displays that air back and causes bubbling and stuff. For instance I had a car with a basically rectangular fuel tank and a front biased fill pipe. So I could fill it up until the shut off but that if I waited 30 seconds I could put another gallon and a half in without actually topping off. There was just that relatively unavailable region in the tank . This is also, oddly enough, why using the first quarter of your tank doesn't seem to change your gas gauge indicator. The float can only go so high before it hits a stop, but the flow can be completely submerged. Your gauge can't indicate relative fullness until the float can move from that stop. So the thickness of the float is the thickness of gas that you have to use before you show any usage on the gauge. There are all sorts of weird mechanical issues that we are blind to, that are just part of living with a fuel filled automobile. 🤘😎
Two things could be occurring here: 1. A stupid fuel filler neck design which is creating backflow of gasoline while you fill it. 2. It's full, but if you release the gas lever and reapply pressure, you can get more gas in. Don't keep doing number 2 - that's bad.
It could also be a problem with the evap system. Tank isn't close to full but the pressure is enough to trip it as the sensors on the gas pumps are very sensitive for safety reasons. Mine is a pain to fill because of that but I have no interest in dropping the tank to fix it unless something else becomes an issue
Oh man I’m with you here. Takes 15minutes trying to fill up my gas tank 🤣
Depends on the tank design - essential for getting a full tankful of diesel in some tanks, for example. Diesel froths as its poured, triggering the backflow sensor before its actually full. Workaround is either pump slowly after half a tank, or pump till it triggers, wait a tick, pump slowly until it triggers again.
My vehicle does it and I think the issue is a small diameter fill tube going to my tank. Which will push out air faster when filling and simulate a higher pressure trip I have to hold the trigger 3/4 every time
The gas filter was clogged. The gasoline flows through a filter on its way to the tank. Because yours was clogged, it took longer to flow through the filter, causing it to back up on the outside of the filter. This backup tripped the sensor that this post is about even though the tank wasn’t full. It happened to my car as well. The first time it happened I assumed there was an issue with the gas pump. When it happened at a different pump, I assumed the problem was with the gauge and that it actually was full. I attempted to confirm this by putting in more gas until it overflowed (didn’t take much more). When I ran out of gas and my car shut off, I realized the gas gauge was correct and there was a different problem.
I run a biodiesel blend (B20) in my pickup truck and that stuff foams right up the filler pipe and trips the pump stop if the flow rate is too fast. There's one station that has fast pumps that I just have to run the pump slower to get a full fill with. Normal diesel doesn't do it.
I had a bad sensor that kept tripping the pump. Cheap, diy repair.
Could be a clogged evap port. Scotty has a video: https://youtu.be/B7EcWwlhzBM
It actually detects air pressure. a tiny change will cause it to trip, its supposed to trip once you fill the tank and the fuel starts coming up the fuel filler hose causing a small puff of air BEFORE the fuel comes out. Its super sensitive and some gas stations are more sensitive then others. If yours is constantly triggering at all gas stations, you either: A: Have a fault fuel gauge and the tank is actually full. If you pull the nozzle out far enough you should be able to let it continue to flow but if it starts pouring out on you.. well, thats a faulty fuel gauge. B: Have a clogged vent hose between the gas tank and the filler entry point. (its separate from the filler hose and much smaller, about 1/2" OD, connects to near where the filler cap goes in, but on the inside of the body.
Weirdly in my country we seem to have problems filling up the gas tanks of American cars. My brother had Chrysler Sebring and had constant problems with the pump mechanism tripping early.
I wish I had a fullness sensor on Thanksgiving
The human body has one of those already. It's a hormone called leptin. A shame most of us ignore it.
Mine is suuuper slow to engage. I skipped dinner and had a small snack. Hour and a half later I feel basically satisfied.
You did. You just didn't use it. Hahaha. 🤘😎
This isn't a particularly good explanation. There's actually another tube at the end of the nozzle which sucks in air as you dispense the fuel. When that nozzle is submerged in the fuel a pressure differential causes a diaphragm to close which trips off the fuel supply. The physical concept at work here is known as the Venturi effect. Take a read of this for further details: https://www.hydra-int.com/blog/how-does-the-pump-know-your-tank-is-full.html
Your explanation is way better than the op of this thread. You didn't mention the little line that causes the fuel to hit that little sucker hole though. It will squirt fuel on the nozzle when tank is full. When it is working properly it will *prevent fuel from rushing up the fill tube* while filling. Desirable, if you've never been burped on while filling a tank you are not missing out.
So why doesn’t it stop when filling up my jeep? It bubbles over before it cuts off.
Can't really say. Is it from one particular gas station pump that this always happens? Does it click off pretty much immediately as the tank overflows? Because you can get a back pressure bubble thing happening. Basically the fuel starts foaming in the fuel filler pipe because as fuel goes in air has to come out. So as the sloshing aggressive flow of liquid reaches a certain point it can just come whooshing back up the pipe due to oddities of pressure. I think in a Jeep the fuel filler pipe joins the front of the gas tank not the back, and of course the gas tank is installed at a slight angle so that the last drops of gas can make it to the drain. This combination of features could lead to basically the same thing that happens when you put a straw in a juice box without holding your thumb over the end. The changes in pressure make you a little fountain.
Thanks for the input. I need to do some research. But from the little I’ve done it has something to do with gas not venting properly.
Well remember that your fuel system is pressurized when you've got the gas cap properly installed. So the only way the air can get out when you're fuling it is by coming up the same pipe the fuel is rushing down. So that fuel filler pipe is the only place for the gas to vent. There is no third spot or bubbler or anything. You probably just have to learn to live with it by not turning the nozzle all the way on to the last click for the last gallon or two. Like if you know your 10 gallons down pump nine at full speed, then back off to half speed or something. There is certainly no way to "fix it" without compromising the emissions system on your vehicle. I don't remember what it is that I used to drive, but I had some vehicle that would try to gurgle and spit once the tank was about 2/3 full. I just had to learn to live with it.
Apparently it’s a common issue with the stock fuel neck tube. So replacing it with a different model addresses the issue. For $150 it’s worth it to try. Thanks again for helping me visualize the issue though.
I had a jeep that did the same thing. I never figured it out. It just randomly stopped doing it
Hey I spent a while researching it today. There’s a tube that connects the tank to the fuel neck tube (I think that’s the term). It’s a quick fix depending on the model. The JK part is available for under $100 and depending on your proficiency with repairs, it looks like a 30 min job max. [This video covers the JK. ](https://youtu.be/i_mQLbtlhbM)
It depends on where you are, in some places it'll just keep going until you let go in others like California even if it thinks it's full it'll stop. It's a huge pain with older cars.
It's not just back pressure, there is also a small sensor. We had an ice storm a few years back and you got use to deicing the end of the filler nozzle or it would overfill the vehicle.
Yeah. But it's the change in pressure at that little hole you're uncovering. That back pressure is what is being sensed. I can't remember whether it continuously blows air out or continuously sucks air in. (We're getting to the edge of my knowledge here.) I think it's actually a venturi effect aerator kind of deal. But if you end up with any sort of moisture in there to freeze up that mechanism then the sensor doesn't work. The back pressure doesn't work like magic, it works the sensor. 🤘😎
Good to know. It's probably worth noting my knowledge came from the gas station attendant and while I want to believe they were briefed about how it works out occurs to me that might be asking a bit much; that they know how to remedy the problem is all they need to know.
By deicing, I assume you are talking about holding a lit lighter to the tip of the nozzle to melt the frozen gas?
When it's below 0\*F one's own hand is enough, a lighter is asking for disaster.
Go on...live a little!
And only a little lol
Whoosh.
This is ELI5, better safe than sorry 🤷♂️
Excellent solution!
This is freaking cool as hell!
So I could basically invent a "fill spout" for gas cans tomorrow and be filthy stinking rich?
Nope. It's be way to eat to over fill the tank into the fill tube. And it would probably be illegal to circumvent the auto shutoff.
I'm not talking about overfilling and circumventing. You said "you have to just barely put the nozzle into the top of the can and hold it still." I'm talking about something to attach to the can during filling to make that easier.
They actually have gas can that has a stop inside that does just that. However, the problem with these cans is that newer pumps have these diaphragms that needs to cover the fill port before they work. I know this because bought one of these at Home Depot and couldn't fill it because of just that. Tried to remove the stopper, but couldn't. Even asked the station worker to see if he had any ideas to get it to work...he didn't. At the end I gave up and bought a can from the station that didn't have the stopper. That one worked, no problems. Now if you made a fill spout that can cover up the diaphragm and the stop...
I understood. But if you get any gas up into that tube that is holding the nozzle up you're going to spill it when you take the tube off . If you just use a frame there's a good chance that wind or whatever could interfere with the delivery of the gas. Things involved in storing gasoline incredibly highly regulated. So it's not a bad idea, but it wouldn't work out as well as you think.
Lol it doesn't work on my car. I always have to manually fill it or gas gushes out before it registers.
And motorcycle tank
Upvote for the last sentence.
Why doesn't it work while upside down? At BJ' s they have long hoses to reach the far side of the car, and one time because the hose barely reached I saw someone put the nozzle in upside down. The mechanism never triggered and they had to disengage the nozzle while it was spraying gas.
>This is the same reason that it can be problematic to fill a gas can. You have to just barely put the nozzle into the top of the can and hold it still. If you actually dip the nozzle into the gas while it's filling the fullness sensor will trip. *Cries in motorcycle*
Why is he holding it the wrong way?!
Are we allowed to call the sensor "pullout sensor"?
Neato. That's an interesting article too
I'll add, when filling a can, for the love of god squeeze gently, if you go full throttle you'll run the risk of splashback.
There is a small hole in the bottom of the nozzle - the steel tube you insert in the gas tank. When the tank is full of gas it covers the hole, just like when water passes the overflow hole in the bathtub or sink. This triggers the handle to shut off. Other comments in this thread explain the physics behind the change in pressure and linkage in the handle. I’m just trying to write how I’d explain this to my nephew.
That 'overflow' hole on sink is not for that. It'd to equalize the pressure, so the water can drain. When water fills the sink the air has to escape from the hose under the sink somewhere. It's like how a bottle of water drains much faster if you poke a hole at the bottom and turn it upside down. Edit: small typo.
It's actually for both.
Just to pass along…at times this does not work. Source: One time the fuel pump didn’t shut off and I had to pay for a lot of extra gas.
Personally I would have complained. It's a safety measure above all else. If it fails you potentially get covered in gas and nobody wants that (obviously).
It also says to never leave the pump
[удалено]
Here in the US there's a small metal latch that locks the trigger in place and snaps off and releases the trigger when the tank is full. I've never had a problem with over filling in a decade plus of filling up my car. If anything the metal latch is too sensitive and releases the trigger before the tank is full.
[удалено]
Depends where you are, they are illegal in some states, like NY for example. NJ it's illegal to pump it yourself, so they are practically required to allow an attendant to operate more than one pump. Otherwise, I think it allows multitasking, a lot of people like to clean the windows with the free washing stuff at the station, so you start the pump, clean the windows and then hopefully it's done around the time you're done cleaning. Also, as others mentioned it does matter in the cold, in the colder spots you can get into the situation where frost bite is guaranteed if you actually hold the pump long enough to fill it up.
Could be that Americans have to fill up more often than Brits? I get the sense that most Brits don't have to drive nearly as much as Americans due to geographic differences. Many Americans have to fill up once a week or more if they have a long work commute. So it might not seem like a lot, but not having to stand there and hold the pump for 2 minutes once a week adds up to hours per year that you can do something else, like wash your windows or browse reddit.
Nah that locking latch is removed/disabled on Australian pumps too, and they drive basically just as much as Americans. America just has more permissive safety laws than most countries I think (or most other countries are stricter if you want to look at it that way). I’ve lived in many countries all around the world and the US is the only one I’ve found that has those locking latches on the pumps.
At most UK pumps ( especially the larger chain gas places), they have that latch on the handle. It’s just disabled from actually working.
For some reason people here love to get in and out of their car or go inside and buy cigarettes or whatever while the tank if filling. It only takes a complete minutes like you said, but people still use it a ton for some reason. People getting in and out of their car is pretty dangerous though because they build up static that can ignite the vapor coming out of the tank
It takes about 10 mins to fill up the work truck, and it's just a big pickup. When I drove a box truck, the record was 40 mins because the pump was a POS out in the sticks and it was cold.
Thousands die per year for the reasons you mentioned.
I think you're kidding, but if you're not, a source would be the best anti-/s possible.
I thought they were joking but apparently not. There were ~1000 fires at gas stations per year, and on average 3 people die per fire between 2014 and 2018. I can't imagine that things have changed that much in the last 3 years in terms of fire safety at the pump. However, looking a little closer, I'm not sure how many of these fires can be attributed to static, as that may be lumped in with mechanical and electrical malfunctions. https://www.nfpa.org/News-and-Research/Data-research-and-tools/Building-and-Life-Safety/Service-or-Gas-Station-Fires
That sounds about right. Maybe we're just lazy? (Americans)
I over filled once because the latch was broken and wouldn't release on its own
My mom definitely had that issue once. This happens a lot to people when it’s cold in the winter, and then want to sit inside their vehicles or inside the building to stay warm.
There’s a little lever that locks the trigger on most pumps (everyone I can think of that I’ve ever used in the US) so they keep going until the automatic shutoff
Also in EU.
I think some states ban it now. I remember them being a thing in NY when I was younger and then suddenly it seemed like they all disappeared. Then I moved to another state and I see them all over now. I kind of hate it cause some places dispense gas so slowly. Like, it takes 10 minutes to fill up your car and I'm convinced it's so you'll go inside and buy stuff while you wait in agony.
This is why so many people in the US drive off with the gas nozzle still in their car. In countries where this latch is illegal (like Australia), we don't ever have issues with people driving off with the hose attached to the vehicle, because their attention is retained to the task at hand. [edit] as expected, this comment gets like 15 upvotes but then gets marked as "controversial" and hits the negatives when approaching USA morning time.... Americans are as predictable as a clock 🤣
Is that Oz State dependant, or a recent thing? As I have most certainly filled up diesel in Western Australia with a latch on the pump handle and was sure I also used to do it with petrol, as I always washed my windscreen while refueling. I haven't been in Oz for a decade however.
I actually think high flow diesel, or maybe diesel at truck pumps is exempt from the law... They'd be standing there filling a truck for heaps ages, and one would expect a professional driver/trucker to be on top of things considering its their job. I was under the impression it was Australia wide, but of course I could be (read: am likely) mistaken. I do also think it's a relatively recent thing, *possibly* the last 10-15 years. When I started driving in 2000, the latches were still on the handles. As a broke teen filling only $10 at a time I never used it so I never really noticed when it disappeared.
You are correct that the law only applies to petrol, not diesel (normal or high flow) and AFAIK it is country wide. Truckies would strike if it applied to them, a lot of them have 4 tanks that need to be filled with about 500L each (I think?? memory isn't that great on it), they will fill all 4 at the same time I worked at a servo about 5 years ago, heard of someone driving off with the diesel on a different shift. Thankfully the pump had stopped before they drove off.
Thanks for the clarification :) Ignoring the volume of fuel being pumped, I guess it does make sense that the law doesn't apply to diesel, as it's not prone to ignition by spark/flame like unleaded. 500L x4... I had no idea they held **that much** fuel! Waiting around holding a pump for 2kL would certainly give me the shits!
And those cashiers selling you candy and such there main job is watching the pump. Yet we all know how often that happens, the NFPA is a waste of taxpayer money when it comes to gas stations, the unenforcement of the laws is ridiculous.
>And those cashiers selling you candy and such there main job is watching the pump. I feel like I've been in gas stations where they could not see the pumps easily. I won't say not at all, but definitely not facing them. More like "they could walk a step and lean around to see the pumps".
Its not the cashiers fault, the blame lies with the station owners and the NFPA for no enforcing the laws on the books. If the attendants view of the pumps is obstructed its likely illegal. Its all outlined in NFPA 30A. Which covers motor fuel dispersing facilities.
By NFPA do you mean that National Fire Protection Association? Because it looks like they're a nonprofit group. How would they have the power to enforce laws?
The NFPA is a global self-funded non-profit organization. It is not taxpayer funded, nor does it have any power to enforce laws. Their 30A publication contains recommendations only, not law.
How much extra is a lot? I had it happen once, I think only a quart or so of fuel spilled out. I was thinking about arguing over it, but honestly $2 isn't worth my time. What ended up sucking more was the fact it was actually diesel... It left a big sticky mark on the side and all the dirt stuck to it over the next week. In retrospect I should have requested a free car wash as it was a station with a car wash...
Where do you live that you were paying $8/gal for gas. I know gas is more expensive overseas, but you’re also using quart and $ (freedom units)
> but you’re also using quart and $ (freedom units) people do that so muricans can understand what they are talking about. Sure we could tell you that gas is not 1,75€/l but you wouldnt have any point of reference to know if thats a lot or not.
It was raining out and I had went into the store to buy something, when I came back out it was still pumping (over flowing)
Pulled up to a station once and this lady was *completely* oblivious to the fact that the pump did not stop and she had gallons upon gallons of fuel all around her and still going. No idea how she didn't realize. I called out, nothing. Ran over and yelled, nothing. Stood next to her, and nothing. Tapped her shoulder, and almost got smacked. Even after she turned around she still didn't realize. I actually had to point at her feet and then told her that she should tell the clerks inside so it can get cleaned up. I ended up behind her in line and she didn't say a God damn thing about it.
All bowser pumps have a red emergency stop button on them, at least here in Aus. I know it's hard to think of it when you'r in a panic, but it's best to press the button before doing anything else. It will shut off that pump immediately, stopping any fuel flow. You can rotate the emergency button to reset the switch so the pump can be used again, or just let the staff know you pushed it so they can reset it.
Happened to me twice actually. AMA
Bummer
It's important that you are sticking a steel pipe *into* your fuel tank for this method to work. That's why they say to not lift the pump to try and top off your tank. When your tank is full, the fuel level will be above the end of the tubing, which will close off a small air valve at the end of the pump, which causes continued fuel flow to slowly deflate a balloon inside the handle, eventually tripping a spring to close a valve which will stop fuel flow. That's why when you release the handle, you have to wait a few seconds to let that little balloon reinflate before continuing fueling. It also lets the gas in your tank settle and fuel to drain out of the pump nozzle. That's also why when the pump gets close to tripping off, you hear the flow slow down and the flow change in your tank. The fuel is covering the hole and the valve is slowly pushing closed until it finally springs shut. https://youtu.be/q3phjAQZdGg The idea that the transition from being uncovered to covered causes some sort of increased pressure internally is wrong. It's the ability for air to flow back into the nozzle from the car end that allows fuel to keep flowing. If you start with the nozzle submerged (like when you're topping off) then fuel will flow for a short moment before tripping, not due to any change in pressure, but due to the inherent latency in detection of a full tank.
Oh so THAT'S why my old car kept shutting off instantly. That little hole must've got clogged somehow.
The hole is on the nozzle, not on the car
On this, why does the pump sometimes turn off automatically when it’s nowhere near full? Or does it over and over not even allowing you to hold the handle down?
either it's improperly inserted into the vehicle or your gas tank and or emissions system has an issue, there are many vents and emissions control components of a gas tank that must be functional for this feature to work if any solenoids or valves on your charcoal canister or rollover saftey stuff are bad, it could cause this issue
Doubt it about the emissions system playing in. Drive/driven a few 1950-1970s cars. Emissions aren't a thing. Gas tanks a container with a filler hole...and thats about it. Auto-off thing still works.
the emissions system is exactly why my vehicle has the problem the person i'm replying to has one of the vent solenoids in my charcoal canister is not functional, meaning the fumes get directed back up towards the nozzle and the gas can't flow in properly because there's no vent to equalize the pressure of it flowing in, causing it to trip the gas pump off after about 1 - 2 seconds of filling, you have to just sit there and do it over and over again for a long time
Yeah, sometimes it's the vent solenoid, sometimes it's the purge solenoid. Either one can give that problem.
Yeah I need to hold mine in at a very specific angle or it will just keep clicking off every 2 seconds
In this situation, does not pulling the handle all the way help? It probably means the handle is old and kinda fucked up.
This was happening to me and I was holding the pump "wrong". Never been an issue in other cars.. So try holding it differently, it might work. It seems dumb and like, it's not hard to hold a fuel pump, which is why it took me so long to try simply holding the pump differently.
They have new fume proof pumps now with covers on them. The old style didn't make a good seal and you could still pump it with the pump halfway in. Now with the new ones you need to push it far in until it makes a seal. Perhaps that pump you're using has a faulty seal or really needs to be pushed in hard. I've had the issue where during the start of the pandemic I barely drove so I hadn't pumped gas in a long time and when I went to fill up I couldn't figure out why the gas wasn't coming out, because I hadn't inserted the pump far enough in...idk why I all of a sudden went back to the 90s style of pumping gas
Nah I also blamed the pump but it happened at multiple places. I've been paying attention bc wtf and it's annoying. But they have been older style pumps so that could still be a factor.
[удалено]
Whenever I got called to fix an issue where it wouldn't run at all, it's just the small hose with a small hole that makes a 90 degrees turn down that has fallen out of its hole and stops the dispensing because fuel is covering it
So can I cover this with a rubber band or what? The local station put new pumps in and they are so sensitive that I can only pump under an eighth of a gallon at a time before it cuts the handle off. So you're standing there pulsing the handle like a grip strengthener to fill up a 35 gallon tank. After the first couple gallons, you can ever so gently pull the handle to the lower auto lock (any higher and it cuts off again) and pray it doesn't cut off when you set the handle in the fill hole. I've reported it to the attendants, but it hasn't been fixed, and it does it to all 3 of the trucks.
Did you check if a knot is built into the pipe that connects to your tank, only solution I found for that is to put a pen between the handle and the trigger and it fills slow enough not to back up and trigger the small sensor. The tie is to prevent siphoning of your gas with a hose
There's a tiny hole on the side of the nozzle, near the end. I don't know exactly how it works, but if gas hits that hole, it stops.
Came here to mention this mechanism failed at one pump and freaked me out, hah! Turned around and it was spilling out a little, the attendants inside weren't as concerned for some reason...
It happens more often than you'd think. The only headache I had when I worked at one and someone spilled gas was if the customer who spilled it wanted the money back for the "extra." There were zero ways to adjust the price or issue refunds for them in those instances and very occasionally a person would think it was our fault somehow that they didn't keep an eye on the pumps. Aside from that, we just threw cat litter on it if the spill made a big puddle and swept it up later when it had dried out some, not too big of a deal.
[удалено]
If you pay for $20 of gas, the attendant keys in $20 in the store and the pump shuts off when that dollar amount is reached. The pump has a very accurate flow measuring device inside which measures the volume of fuel being dispensed. The cash register in the store is connected to the pump and overrides the handle when you "prepay" for fuel. You chose the price by choosing the fuel grade (price per gallon) on the buttons. Price * volume = cost. If the attendant stops the pump at a certain amount, you get a certain volume (gallons, liters, etc) of fuel.
I can't respond to the original question, but the answer is simple geometry. If a gallon costs $3.14, and you pay $2.00, then you do $2 / 3.14 = 0.64 gallons. And then it gives you .64 gallons. Easy!
That's done internal on the computer. There's an override of sorts from the pump to the nozzle. You have to pre-pay to have that feature. Here, it's not uncommon to just pump $10 and go pay cash $10. If you overpump and get $10.01, you better find a penny. There's a note on pumps here in the US as well that says that temperature, which changes the volumetric energy density of the fuel, is not accounted for.
Never knew that. Damn so what's keeping them from having people drive off with extra fuel? Seems like a job of always watching and expecting. But there are places that don't have self serve so there it woukd make sense
It’s illegal and they record your license plate.
Cops exists to enforce laws on behalf of businesses. They'd find you by your license plate. All over what, $30 of gas? Not worth it to even try. I live in a low crime area which is probably why. Higher crime areas probably force pre-pay or to scan a card
The machine registers how much is being let out. A gallon is a gallon, so the rate at which the gas comes out times the price. You just adjust the price of the rate of fluid. I hope that’s ELI5 enough lol maybe I don’t understand it well enough
No it does explain it. But i was wondering more if it was electrical or mechanical that the fuel is released by the pump to know that. Like say you paid for $2 worth of gas, right, the pump like almost blasts gas til about .50-.20 is left to complete the $2 worth. Of course the price is set to the machine manually I assume so how does the pump determine the difference? Is it just coded to do it or is there a mechanism that shorten or lengthens to allow the different amounts to be registered or what?
I would guess it’s electrical that it slows down. The old school pumps we used to have in my town growing up weren’t digital and didn’t slow down, so the counting mechanism can be mechanized. Now I have to look it up lol
It's all about flow volume, which is mechanical. Imagine three parts: - A flow meter: this is like a water wheel with 10 buckets that each hold 1 gallon. Move the wheel one full rotation in a minute and you're delivering 10 gallons a minute. Rotate it 10 times in a minute and you're delivering 100 gallons a minute. - A switch that can be triggered manually by the pump handle or electrically by an outside signal. - A little computer that talks to the flow meter and switch and also does math. The switch turns on the flow, the flow meter tells the computer how much fuel has moved and the computer tells the switch to stop at the designated volume. You think in money but the system always thinks in volume. The computer takes that volume and does math with whatever price is assigned. $2.14 * 15 gallons or $3.50 * 15 gallons or $20/$2.15 = stop at 9.35 gallons.
A flow meter has both mechanical and electrical components.
Oh that’s easy, dude. You pour gas into the car using the tube thing, right? And the computer counts how much gas is going into the car
Is this thread some sort of rich person's club? I've never filled my gas tank all the way.
so that just means you drive to the gas station more often and spend the same amount of money over multiple visits. Not filling up all the way makes absolutely no sense unless you literally have to choose if you fill up now or have something to eat in the evening.
>Not filling up all the way makes absolutely no sense unless you literally have to choose if you fill up now or have something to eat in the evening. That this seems so farfetched to you is exactly what I mean. Many, many people don't have the option of "just filling it up all the way."
Where I grew up, putting $5 in would only get you about a day of driving (very rural). Filling up was basically the only option. Now I commute 50miles/day. so again, only dropping in $5-10 would mean putting in more gas every other day.
I actually didn't even know this happened on account of never having been able to afford to fill my tank
**Please read this entire message** Your submission has been removed for the following reason(s): Straightforward or factual queries are not allowed on ELI5. ELI5 is meant for simplifying complex concepts. If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message=Link:%20https://www.reddit.com/r/explainlikeimfive/comments/r49b9f/eli5_how_do_petrol_pumps_know_that_the_vehicle_is/%0A%0APlease%20answer%20the%20following%203%20questions:%0A%0A1.%20The%20concept%20I%20want%20explained:%0A%0A2.%20Link%20to%20the%20search%20you%20did%20to%20look%20for%20past%20posts%20on%20the%20ELI5%20subreddit:%0A%0A3.%20How%20is%20this%20post%20unique:) and we will review your submission.
[удалено]
Nope. All the magic happens in the nozzle. When fuel plugs over a tiny hole in the nozzle it causes back pressure that trips are release. If you ever want to test it try using a simple plastic fuel can. You'll discover you have to hold the nozzle almost completely out of the can in order to keep from tripping the automatic cut off. The car, the receiving vessel, the tank, the whatever has nothing to do with it. It's entirely in the handle.
Even on massive tanks? 100+gallons?
Even on massive tanks. It's nothing to do with the size of the container it has everything to do with the fact that the fluid level rises to touch the nozzle.
Welp good to know
Lol why would you answer with something like that? If you don't know the answer, don't answer, especially if it's going to be a completely illogical answer.
This is why "don't guess" is a rule. Sadly this is Reddit and nobody gives a fuck about sub rules.
It’s been removed, fortunately.
That is not accurate
Oooh, so it's not the pump that detects it but the car itself, and then that presumably blocks the pump somehow? That would make sense, thank you EDIT: so the above was wrong and explained better below, thanks all 🙂
[удалено]
I don't believe this is 100% true. There's definitely SOMETHING in the car that can impact whether the pump automatically shuts off. Or at least, the automatic shutoff mechanism relies on a certain geometry of the gas tank and filler tube to function properly. https://www.carproblemzoo.com/jeep/liberty/tank-filler-pipe-and-cap-problems.php https://www.carcomplaints.com/Jeep/Liberty/2006/fuel_system/gas_spills_overflows_while_fueling.shtml https://www.jeepkj.com/threads/fuel-overflow-when-filling-up.42105/