# How does the gas pump not pop out? **URL:** \ **Category:** Factual Questions\ **Created:** [July 30, 2009, 6:50pm UTC](https://boards.straightdope.com/t/how-does-the-gas-pump-not-pop-out/504800 "2009-07-30T18:50:41Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![YogSothoth](https://avatars.discourse-cdn.com/v4/letter/y/8edcca/32.png) [@YogSothoth](https://boards.straightdope.com/u/YogSothoth)\ **Post date:** [July 30, 2009, 6:50pm UTC](https://boards.straightdope.com/t/how-does-the-gas-pump-not-pop-out/504800/1 "2009-07-30T18:50:41Z")
The gas pump is a metal cylinder surrounded by this rubber, springy thing. Usually, I shove the metal into the gas tank as far as it will go and then…nothing. It doesn’t pop out. The springy rubber thing does nothing. All I have to do is flip that little metal stub to keep the trigger down and I can walk away from the pump and let it fill up automatically. How does it do that without popping out? In my decades of driving, that has never happened, to me or anyone else near me who’s buying gas. There’s no hook or catch on the metal cylinder or inside the gas tank, otherwise I could see it or it would occasionally get stuck. It just magically stays in. Another question: How does the pump know you’ve had enough gas? Some kind of pressure sensor? If its a sensor, wouldn’t any gas filling up the tank trigger it since the rubber thing keeps the area sealed? ---
**Author:** ![Jas09](https://avatars.discourse-cdn.com/v4/letter/j/d07c76/32.png) [@Jas09](https://boards.straightdope.com/u/Jas09)\ **Post date:** [July 30, 2009, 7:48pm UTC](https://boards.straightdope.com/t/how-does-the-gas-pump-not-pop-out/504800/2 "2009-07-30T19:48:21Z")
The gas pump stays in via gravity. It works the same whether the “rubber thing” is present or not (and they are relatively new additions to gas pumps). The springyiness of the rubber housing is not sufficient to overcome the gravity pulling the pump nozzle down into the tank (and being held into place by resting on the top of tank tube). The autocutoff is done via a second sensor tube connected to a Venturi pump. You can read more about that [here](http://en.wikipedia.org/wiki/Gas_pump#Autocut_in_fuel_dispenser). The rubber housing does not create an air-tight seal, it merely helps collect a majority of the vapors. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [July 30, 2009, 7:54pm UTC](https://boards.straightdope.com/t/how-does-the-gas-pump-not-pop-out/504800/3 "2009-07-30T19:54:05Z")
Just be careful - you don’t want to get hurt in a freak gasoline-fight accident. ---
**Author:** ![YogSothoth](https://avatars.discourse-cdn.com/v4/letter/y/8edcca/32.png) [@YogSothoth](https://boards.straightdope.com/u/YogSothoth)\ **Post date:** [July 30, 2009, 7:55pm UTC](https://boards.straightdope.com/t/how-does-the-gas-pump-not-pop-out/504800/4 "2009-07-30T19:55:57Z")
> [@Jas09](#): > > The gas pump stays in via gravity. It works the same whether the “rubber thing” is present or not (and they are relatively new additions to gas pumps). The springyiness of the rubber housing is not sufficient to overcome the gravity pulling the pump nozzle down into the tank (and being held into place by resting on the top of tank tube). But the back end of the pump is larger and heavier than the little metal tube you stick into the gas tank. How does gravity not pull it out? ---
**Author:** ![Nunzio\_Tavulari](https://avatars.discourse-cdn.com/v4/letter/n/b9bd4f/32.png) [@Nunzio\_Tavulari](https://boards.straightdope.com/u/Nunzio_Tavulari)\ **Post date:** [July 30, 2009, 7:57pm UTC](https://boards.straightdope.com/t/how-does-the-gas-pump-not-pop-out/504800/5 "2009-07-30T19:57:05Z")
The metal nozzle is fairly heavy, and the stiffness of the hose acts like a spring that resists the force of the gasoline coming out. If you hold onto the nozzle when pumping, you’ll notice that there’s very little kickback when you start the flow, so the pressure probably isn’t very high. My guess on the nozzle shut-off is that gasoline flowing through the nozzle encounters little resistance as it pours through the air in an empty fill tube. But once the tube fills up, you are trying to pump gasoline where it already exists. It can’t be compressed, the pressure sensor senses difficulty and shuts off. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [July 30, 2009, 8:33pm UTC](https://boards.straightdope.com/t/how-does-the-gas-pump-not-pop-out/504800/6 "2009-07-30T20:33:15Z")
> [@YogSosoth](#): > > But the back end of the pump is larger and heavier than the little metal tube you stick into the gas tank. How does gravity not pull it out? Usually, the heavy end outside the tank hangs down, pivoting at the point where the nozzle rests on the filler tube, pulling the end nozzle against the side of the filler tube. The friction there helps keep it in place. > **[Stock Photography - Search 45.4 million Stock Photos, Stock Footage Video...](https://www.fotosearch.com/)** > > Fotosearch Stock Photography and Stock Footage helps you find the perfect photo or footage, fast! We feature 45,500,000 royalty free photos, stock footage clips, digital videos, vector clip art images, clipart pictures, background graphics, medical... Here’s an illustration that might help. The weight on the handle should help the nozzle stay in place. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [July 30, 2009, 9:04pm UTC](https://boards.straightdope.com/t/how-does-the-gas-pump-not-pop-out/504800/7 "2009-07-30T21:04:56Z")
The rubber seal isn’t part of the automatic shut off. It’s for evaporative emissions recovery, and prevents gasoline fumes from escaping into the air. The rubber springy thing is designed to be just springy enough so it that it makes a reasonable seal no matter how far in or out you have the nozzle. It’s not designed to have enough strength to pop the nozzle back out. Pumps can have an automatic shutoff without having the rubber springy thing. The automatic shutoff works on back pressure, which you can easily demonstrate on some vehicles. Sometimes you get a combination of the bend in the filler tube and the bend in the nozzle disagreeing a bit with each other, so that when you try to fill up it doesn’t line up right and the added back pressure causes the automatic shutoff to keep shutting off. It’s easily fixed by just pulling the nozzle out slightly or just jiggling it around to put it in a position where you don’t get so much back pressure. Don’t ever “top off” your tank when filling up. If you try to squeeze too much gas into your tank, it can end up going into the pump’s emissions recovery system, and it can be forced down into your car’s emissions system as well. You can easily end up with gasoline in your charcoal filter, which will usually cause the car’s “check engine” light to come on. In many states this fails the car’s yearly inspection, and you’ll have to get the charcoal filter replaced (to the tune of a couple hundred bucks) before the car will pass inspection and be legal again.