Why do gas pumps shut off mid-fill?

April 25, 2005 11:15 AM

When I gas up my car, I don't squeeze the lever on the pump handle, but, instead, I slide the lever into the little notch so that my tank will fill itself up while I squeegee the windows. When the tank is full, the pump shudders, the lever dislodges itself, and the filling ceases. However, probably half of the time, the filling ceases much, much too early. The result is that I can't tell when my tank is full, nor can I squeegee while my car fuels itself. What gives?

My car is a 1994 Volvo, but I've had the same experience with other cars. I don't know if the problem is with the gas station's equipment or with the car -- that is, is my car triggering the nozzle's "hey the tank is full" mechanism falsely, or is the nozzle at fault? Can I twiddle my car's tank, or is it possible that I could find a gas pump that doesn't have this problem? It's really annoying -- particularly after I drive off to find that my tank is only 3/4 full.

posted by waldo to travel & transportation (25 comments total)

This is perfectly normal. It happens because as the fuel goes into the tank it splashes and bubbles. As the tank gets closer to being full, the displacement caused by the bubbles forces the gas to go up the pipe that feeds the tank and triggers the "full" switch on the pump nozzle. This is similar to if you are filling up a cup of cola at a restaurant, you have to wait for the fizz to die down before you can put that last 1/4 in.

That still gives you lots of time to squeegee, you just need to squeeze out that last bit yourself at the end. No biggie.
posted by furtive at 11:23 AM on April 25, 2005

I believe what you're seeing is a feature, not a bug. Those holding pumps are, in some jurisdictions, considered just too darned dangerous for self-serve users to use. They're supposed to cut out early, forcing you to top the tank to avoid any possibility of overfilling while you're not paying attention.

Set it to pumping, squeegee, then come back and pump it some more manually to fill out your tank.
posted by jacquilynne at 11:24 AM on April 25, 2005

This afternoon, I had a bone-dry tank. I put in $2 worth and -- kachunk it stopped. Another $1 -- kachunk. $3 -- kachunk. $0.50 -- kachunk. Etc. It took a dozen successive squeezes until I got bored and figured it was full enough. I drove off with my tank at 4/5, and resolved to Ask Metafilter™.

Your explanation would make sense if the tank were getting more full. But an 18 gallon tank with 1 gallon in it?
posted by waldo at 11:25 AM on April 25, 2005

Old Straight Dope article on how the auto shut-off mechanism works. Basically if the gas is foamy, they shut off early. I know I've heard somewhere that this is more likely to happen with an empty tank.
posted by cosmonaught at 11:29 AM on April 25, 2005

Your explanation would make sense if the tank were getting more full. But an 18 gallon tank with 1 gallon in it?

Well now you're changing your story, before you said it happened at 3/4, not 1/18th. I've driven and fueled up various marks of car, jeep, truck and tank and I've never had it stop after just a bit. I'm not saying it can't nor doesn't happen, but I'm pretty sure that's the exception to the rule.

Perhaps someone can shed some light as to what causes the fuel pump to stop (I'm guessing a change in pressure on the flow).
posted by furtive at 11:30 AM on April 25, 2005

I've found a similar thing in the UK. I think the petrol comes out of the nozzle and bubbles a little as it sloshes down the pipe into the tank. When the bubbles hit the sensor on the end of nozzle, the pump shuts off. If I wait and listen for the gurgling and bubbling noise to subside, I can then fill the tank to the top.

I recall there being some sort of vacuum sensor on the end of the nozzle. Whilst the tank is filling the small tube is free to draw air in. When the petrol covers the end of the tube, the pump shuts off. I've had a rummage for a design of it but google-fu is falling short at the moment...
posted by gaby at 11:30 AM on April 25, 2005

Looks like cosmonaught beat me to it, and proved my point.
posted by furtive at 11:30 AM on April 25, 2005

On my Mazda Protege, I've had this happen with one particular pump at a local gas station. It would cut out after only a gallon or two. I would walk away just to hear it go kachunk, pissing me off royally. The only way I found to fix it was to rotate the handle before I inserted it. Instead of being stright up and down, more like 45 degrees. (I usually just go to another pump though.)

I figure it is due to the shape of the gas tank near the inlet. On your car, the fuel is splashing back off of something and triggering it to turn off prematurely.
posted by smackfu at 11:32 AM on April 25, 2005

Well now you're changing your story, before you said it happened at 3/4, not 1/18th.

Not changing -- I was just unclear. :) The reason that I drive off with a 3/4 full tank is because I find it very difficult to estimate when the ka-chunk means "your tank is full" and when it means "hey, I'm turning off now, for no reason whatsoever." Rather than overfill my tank, I overestimate, and, hence, the 3/4 full tank.

cosmonaught, that Straight Dope link is great -- thanks for that.

The only way I found to fix it was to rotate the handle before I inserted it. Instead of being stright up and down, more like 45 degrees.

Ah-ha -- that's just the sort of clever hack that I was looking for. I'll certainly try that next time.
posted by waldo at 11:35 AM on April 25, 2005

Well, this sort of covers it. It's a dual valve design. The explanation could be a little simpler but it's there. Also found this on groups:

The flow of fuel through a venturi in the valve draws air from a small hole near the end of the nozzle. When the tank fills, gasoline is drawn up by that small hole. A hydraulic mechanism senses the difference in density and trips the nozzle handle release, thus stopping the flow.
posted by gaby at 11:36 AM on April 25, 2005

On many pump handles, they have several notches that you can slide the lever into, resulting in successively slower filling speeds. Choose the one that pushes the handle in the least, and you will probably have fewer premature shut-offs.
posted by MrZero at 11:38 AM on April 25, 2005

This afternoon, I had a bone-dry tank. I put in $2 worth and -- kachunk it stopped. Another $1 -- kachunk. $3 -- kachunk. $0.50 -- kachunk. Etc. It took a dozen successive squeezes until I got bored and figured it was full enough.

98% of the time my car fills up normally--it shuts off only when the tank is full. 2% of the time something like what you describe happens. My solution is to remember which pump it happens at and not use that pump again--or more drastically, not to use that gas station anymore. (If they can't bother to ensure their pumps are working properly, they must not want my business that badly.)

Is this something that happens frequently, at lots of different pumps at lots of different gas stations? If so, the problem's probably at least partly with your car. If it only happens occasionally, I'd assume the problem's with the pump.
posted by DevilsAdvocate at 11:42 AM on April 25, 2005

Has your car ever been in an accident? I had the right rear quarter panel of my car crunched by a cable spool (don't ask), and after it was repaired I experienced this problem. I think that the filler tube running to the gas tank got kinked.
posted by Daddio at 11:48 AM on April 25, 2005

This happens to me constantly on my 79 Econoline. in fact, I can't even use the little notches to fill the tank, I've got to sit there and finesse the trigger to get it to pour in regularly, and hold it at just the proper angle -- wrong angle: shut off; too much pressure: shut off. Same problem with a little less severity on my 64 bel air.

My guess is that it's due to the way the valve or whatever on your car is shaped -- maybe that particular model volvo just has a clumsily engineered opening (because one would assume the designers would have been aware of the auto-shut off technology by 1994).
posted by fishfucker at 11:53 AM on April 25, 2005

On many pump handles, they have several notches that you can slide the lever into, resulting in successively slower filling speeds. Choose the one that pushes the handle in the least, and you will probably have fewer premature shut-offs.

I had the exact same theory, but as early as this afternoon, no dice.


My solution is to remember which pump it happens at and not use that pump again--or more drastically, not to use that gas station anymore.

That's what I've been doing for the past year, but, embarrassingly, I'm completely incapable of remembering which pumps at which stations worked flawlessly last time I was there. I need a little notebook or something. :)


Has your car ever been in an accident?

Not in a relevant portion of my car, but this perfectly explains why I had the same problem on my family's previous Volvo.
posted by waldo at 11:53 AM on April 25, 2005

Not in a relevant portion of my car, but this perfectly explains why I had the same problem on my family's previous Volvo.


there you go. BAD DESIGN. BAD VOLVO. YOU MAY BE SAFE BUT I CANNOT AUTOFILL YOU AT THE PUMP.
posted by fishfucker at 11:54 AM on April 25, 2005

i heard once, on Car Talk I think, that this can happen to a car when the tank gets over filled - the air thing gets filled with gas and gets confused about whether it's full or not. They said this is the reason you shouldn't top off your tank to the top. Of course, this is just a memory of a radio show from 4 years ago or so, so i'm not totally sure if i have the details right.
posted by muddylemon at 12:07 PM on April 25, 2005

Just a note on over-filling. According to my mechanic friends, it can cause your "check engine" light to come on as well as starting the car with the gas cap off. If memory serves, it was that over-filling and the cap off messed with the oxygen sensor which in turn triggers the light.
posted by ..ooOOoo....ooOOoo.. at 12:09 PM on April 25, 2005

This afternoon, I had a bone-dry tank. I put in $2 worth and -- kachunk it stopped. Another $1 -- kachunk. $3 -- kachunk. $0.50 -- kachunk. Etc. It took a dozen successive squeezes until I got bored and figured it was full enough. I drove off with my tank at 4/5, and resolved to Ask Metafilter™.

This happens to me once in a while. My husband says that this happens when the tanks are getting empty. (the station tanks, not your car tank)
posted by krisobi at 12:25 PM on April 25, 2005

I used to have the same problem, as well. Canadian pumps tend to have three or so little notches that you can leave the pump on, with each successive notch filling your tank more quickly. I found that the pump was less likely to prematurely click off when I left the nozzle on one of the slower notches. Once the pump clicked off, I would then fill the last little bit of the tank by hand (generally about $1 worth of gas).
posted by lumiere at 12:35 PM on April 25, 2005

The surest way to avoid driving away from the gas station with a partially filled tank (when you meant to fill completely) is to actually pay attention to how much gas is going into your car. Like if my 11-gallon tank is half full, I wouldn't expect it to be completely full if I've only put a dollar or two into the tank. This seems really obvious to me.

Or you could always move to New Jersey.
posted by elisabeth r at 12:37 PM on April 25, 2005

I managed a gas station for 3 years. This does not indicate that the pumps are working incorrectly, and there is no relation to how full the station's tanks are. If either of these were true, several successive cars would have the same problem, and they don't. So it's the car, and not the pump.

It is one of two things
a) the bubbling thing that has been explained very well above (think of pouring juice too fast out of a TetraPak - it glugs so the air can escape)
OR
b) the combination of the shape of the bend on the nozzle not fitting quite right with the bend in the pipe in your car that goes to your gas tank.

I have always been able to fix this by not inserting the nozzle as deep into the car. Try just pulling the nozzle out of the car a tiny bit (like 1/2 inch). It seems to allow the gas to flow down into the car more smootly and the displaced air to escape more easily. This has fixed the problem for me every single time.
posted by raedyn at 1:42 PM on April 25, 2005

I'm with the people who have had it happen with a particular pump or a particular station. Weekend before last, I had to fill up at a pump that simply would not let me latch it. If I tried it would kick off within a few seconds. Had to stand there manually holding the nozzle almost all the way to the first latch and swearing every time it still kicked off.

This happens to me once every year or so, when I'm traveling, never at a gas station I regularly go to. Perhaps, since they're out in the boonies, the pumps get maintaned somewhat less recently than pumps in the city.

I'll remember the angle and/or pullout trick to try next time.
posted by kindall at 1:52 PM on April 25, 2005

It definitely depends on the car. My Toyota's tank is waaay forward of the filler, so a long hose connects the two. The hose backs up and stops the pump about 3 gallons before the 13-gallon tank is full. It takes soooo long to jiggle, trickle, and cajole those last gallons in, because of the paranoid stop mechanism on the pump.

I've reduced the problem by parking the wrong way at the pump. My favorite station has their whole area sloped away from the building for rain runoff, but combined with the traffic flow and the fact that my filler's on the driver's side, it means the filling hose is always uphill. Parking the wrong way puts the hose downhill, and I only have to trickle in the last gallon or so.
posted by Myself at 5:09 PM on April 25, 2005

Agree with raedyn: whenever I get a sensitive pump (or whatever it is), I pull the nozzle out so that it's barely in the gas tank. This usually solves the problem.
posted by Hankins at 6:08 PM on April 25, 2005