Why Your Gas Pump Keeps Clicking Off Before Your Tank Is Full

As we enter the dog days of summer, there is a curious side effect of the heat that you'll notice without realizing it. You'll see it as more of an irritation than a science-backed phenomenon.

It's at the gas pump.

Specifically, when the gas pump clicks off, and when you get back in your car, it's still not full.

Casey's is not trying to literally gaslight you. It's just the heat of summer working against this ancient technology.

How a Gas Pump Knows Your Tank Is Full

Gas pumps, while an old design, are still pretty genius. The handle only works as long as a vacuum is pulled on the nozzle. When the tank gets full and the vacuum stops, the pump automatically shuts off.

It's a super-simple design from decades ago, but it works and keeps us from overfilling our tanks.

So why does it cut off early during hot weather? It's the hot weather.

Gasoline acts like most other liquids. When it cools, it shrinks. And while the gas station tanks may only be a few feet under the parking lot, the temperature difference is very noticeable when it's 103° up here on the surface.

As gasoline warms, it expands. Pumping cool gas into your hot gas tank speeds that process up.

If you scroll TikTok or Instagram enough, you've seen this in all of those conspiracy gasoline pump videos where they swear pumping gas at the fastest setting is ripping you off... It's not. It's just a slightly faster evaporation, and that causes more bubbles.

Bubbles can feed backward up your gas tank's filler neck and kick the nozzle off. No big deal, you can always just click it back on and continue. What you lose through evaporation is measured in milliliters, an incredibly small amount of gasoline.

So if you put the pump up after that first nozzle click, you might not have a full tank.

Why You Should Never Top Off Your Tank

On the flip side of that, if you keep forcing gasoline into your tank, it's actually really bad for your vehicle.

Remember, it's been stored underground at a relatively cool temperature. Now that you've filled your tank as full as it can get, when you cap it off and drive on down the road, the heat can make that full tank of gas expand.

It's not enough to make your tank pop like a hot can of beans on a stove. You're likely not going to rupture your gas tank, but it is enough to start messing with your electronics.

It has to go somewhere, and modern vehicles are designed to retain all of that fuel. It can flow into any other pipe that is connected to the tank, most of which have electronics packed into them also. Eventually, the check engine light comes on, and fixing that problem can mean replacing fuel vapor system components that were never meant to be soaked with liquid gasoline.

Shoutout GMT800's.

Moral of the story? Try to trust your intuition. It's far better to get 9/10ths of a tank of gas rather than forcing 11/10ths into it by accident.