How Does My Gas Tank Know When It’s Full?

Anyone who has ever filled up a gas tank has experienced the tactile "clunk" the nozzle makes when the fuel tank is full. This sound comes from the nozzle right when the flow of fuel stops. Most people barely take notice, brushing it off as another little convenience that the world is full of. For those who wonder how the pump knows how much fuel is in the tank, the truth is inevitably much more simple (and ingenious) than they might think.

Why overfilling the gas tank is bad

Gasoline makes vapors that are hazardous to humans in a number of ways. The vapor hangs around and reduces air quality. On top of the fact that it makes breathing harder, fuel vapor is also highly volatile, and is responsible for many fires and explosions each year. In the past, gas caps vented the vapors into the air. This would be fine if humans didn’t insist on breathing so much; but since that’s not the case, a better solution was needed.

Enter: the fuel vapor canister. This neat little innovation is basically a can full of carbon (like an aquarium) that filters the vapors from the fuel tank and allows the gas to re-enter the fuel system, improving fuel efficiency, safety, and air quality all at once. This also regulates pressure in the tank.

What happens if there's too much fuel

The outlet through which the excess vapors leave the fuel tank is found in the filler neck. If too much fuel goes into the tank and fills it up along with the filler neck, then liquid gasoline will enter the canister. Since the canister is made to handle vapor only, this wreaks havoc on the carbon inside. Sometimes the entire canister has to be replaced after it is flooded.

To keep this from happening, there is a small tube running up the length of the nozzle that exits just below the main opening. This tube sucks in air. This allows the nozzle to get a good seal on the tank when inserted into the filler neck by removing the air displaced by the fuel entering the tank. This tube has a narrow section just a few millimeters long called a venturi valve. The narrow section slightly constricts flow and allows the sections of tube on either side of the valve to have different levels of pressure. As soon as gasoline reaches the inlet at the end of the nozzle, the vacuum caused by the higher-pressure air closes a valve and halts the flow of gasoline.

Unfortunately, some people try to get around this by pumping more gas into the tank after the valve shuts. They may even lift the nozzle further out of the filler neck to prevent the venturi valve from doing its job. This, best case scenario, adds a negligible amount of gas while causing a little bit to be sucked back into the nozzle with every clunk and, worst case scenario, causes fuel to spill out of the tank.

Avoid pumping more gas after the valve in the fuel pump nozzle shuts once. The tank is sufficiently full.


The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.

GET A QUOTE

Related articles

Symptoms of a Bad or Failing Fuel Rail Sensor
Common signs include the Check Engine Light coming on, the car not feeling right when running, and issues with starting the car.
How Long Does a Fuel Injector Last?
The fuel that resides in your gas tank has to be carried to various spots of the engine to be burned and used...
How to Know What Type of Gas to Use
Your Your vehicle is propelled by a combustion engine. That means that fuel is injected into the engine’s cylinders in some fashion, where it combusts, or burns, to create downward force against a piston in the cylinder, turning a crankshaft...

Related questions

Car intermittently won't start
Hi there. An intermittent no start condition, can be difficult to diagnose. Replacing the fuel filter (https://www.yourmechanic.com/services/fuel-filter-replacement) would not hurt anything, especially if it hasn't been replaced in the last 30,000 miles. Diagnostic procedures should be performed, at the time...
cranks but wont start
There may be a downloadable diagnostic trouble code(s) (from the PCM) so I recommend that you request a no start diagnostic (https://www.yourmechanic.com/services/car-is-not-starting-inspection) to first see if there is a quick and ready indication of the problem and thence a solution....
Replaced fuel pump in tank. But still no gas getting to carb. Gonna check filter under bed in morn to be sure it's working
With a fully charged battery, if the pump volume and pressure at the tank outlet is normal, but pressure falls off at the delivery point (carburetor, fuel rail), there is a restriction somewhere in the line and/or in the fuel...

How can we help?

Our service team is available 7 days a week, Monday - Friday from 6 AM to 5 PM PST, Saturday - Sunday 7 AM - 4 PM PST.

1 (855) 347-2779 · hi@yourmechanic.com