The mysterious gasoline-feeding apparatus that fills our cars with fuel at gas stations knows exactly when to stop at a full tank with a signature, satisfying click. Have you ever wondered how this works? The theory is simple, but I saw people on Twitter mention that they didn’t know how it works, so consider this a quick explainer.

To start, you need to know your fuel tank has fuel and air in it. Thanks to vents and breathers that are usually controlled with purge valves and other evaporative emissions equipment, the tank can self-regulate the amount of vacuum or pressure that exists inside of it. Ideally, a gas tank has the same pressure as the atmosphere outside and a proportionate amount of air enters the tank as the car uses fuel on a drive.

When the time comes to fill the car up, you open the fuel cap and a little bit of pressure may escape from the tank, which is what that hissing sound is. Either way, now it’s somewhat open to the outside world. This is when the gasoline pump comes into the picture. While the gas pump is filling the tank with fuel, air is pushed out of the tank because gasoline is taking its place. 

At the tip of the pump nozzle, a small port is cut into it for some of that displaced air to travel through, which travels through a dedicated tube to a diaphragm that gets pumped up like a balloon by that air, helped along by a Venturi in the handle of the pump that draws the air through the tube. As long as that diaphragm is pumped up, the gas pump will continue pumping because it is physically connected to a shutoff valve in the handle of the pump. Ever notice a raised dome section on the top of a gas pump handle? That is where the diaphragm is.

Once the car’s tank is full of fuel, a lot less air is going to be displaced thanks to the much smaller volume of the fuel neck and fuel will start to travel up the filler neck to the vacuum port at the tip of the gas nozzle. This reduction in rushing air or replacement by fuel allows the diaphragm to deflate, which activates the shutoff valve. It’s that simple. 

If you’ve ever seen the sign at gas stations that say “do not brim the tank” or something similar, there is a good reason beyond preventing spills. Most modern cars have a charcoal canister near the top of the filler neck. This is designed to capture that fuel-air mixture that comes up the filler neck and recycles it back into the evaporative emissions system. If fuel floods the charcoal canister, it ruins it fairly quickly and prevents it from working correctly and can damage other emissions components.

It depends on the length of the nozzle on the gas pump, but it should be long enough to detect the gasoline before it makes it to anything important. Gas stations all have slightly different nozzle styles that contribute to minor changes that makes this possible. It’s for the good of the car and reduces the risk of a spill. 

It’s an ingenious bit of technology that isn’t anything complex. It’s been around for decades and is reliable. Now you know that there isn’t any magic stopping a gas pump from overfilling your car.