Why Can’t I Top Off My Tank?

Nearly everyone who has been behind the wheel of a vehicle has pumped gas into the fuel tank until the tank was full. When the tank is full, the nozzle makes a ‘clunk’ sound and the gas stops flowing. The nozzle is able to determine when the tank is sufficiently full and shuts off the flow both for safety reasons and for the benefit of the fuel vapor canister.

Even though it seems unlikely to have a part in the fuel system that is specifically not meant to hold liquid fuel, the fuel vapor canister plays an important role that most people don’t even know exists.

When gas is a gas...

Gasoline is usually abbreviated as just ‘gas’. This is a funny nickname, given that a gas is by definition not a liquid; but ‘gas’ is a liquid… or is it? Gasoline produces vapor that is explosive, toxic, and highly volatile. A cloud of vapor is also a lot harder to spot than a puddle of gasoline, so it is much less visible while being significantly more dangerous.

In the past, cars have relied on gas caps that allow the vapor to vent from the fuel tank to prevent pressure from building up. While this protected the fuel tank, the environmental consequences were severe. Gas vapor is an unpleasant gas that negatively affects breathable air, and is a big contributor to smog and poor air conditions in urban areas.

Unsurprisingly, modern cars have a much better solution to the problem of gas vapor in the fuel tank; and that solution is a fuel vapor canister. This neat little part is filled with carbon, like the filter in an aquarium, which keeps vapors in check and allows the vaporized gas from the tank (and carburetor if applicable) to re-enter the fuel system and get burned up in the engine. Liquid fuel reduces the effectiveness of this canister and can cause enough damage to render the vapor canister completely useless. This will lead to an expensive repair but it can be easily prevented.

Do not top off the tank after the valve in the nozzle stops the flow of fuel.

There are many ways to get around the little mechanism in the pump nozzle that stops the flow of fuel once the tank is sufficiently full. Some people keep pulling on the handle, making the pump stop over and over until they are positive it is full. Others lift the nozzle out of the filler neck to prevent the valve from closing. Either way, this practice can get liquid fuel into the vapor canister and can also cause gas to spill out onto the ground.

Spilled gas is an unnecessary hazard, and gas stations go to great lengths to prevent it. Topping off the tank can cause gas to splash out of the filler neck onto the person pumping and the ground around them. The best-case scenario after this happens is a car interior that now smells like gasoline. The worst-case scenario would involve a lot of fire and avoidable insurance claims.

You cannot top off the fuel tank because the fuel pump nozzle will alert the person pumping when there is a sufficient amount of gas in the tank by shutting off the flow. Even though it is possible to add more gas after this point, it is not advisable.


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

How to Replace a Fuel Gauge Sender
The fuel gauge has a sender that goes bad when it says that the fuel level is different lower or higher than it actually is.
How to Fuel a Race Car
Fueling a race car depends on the type, whether NASCAR or Indycar. The process requires a special fueling device, precision, and speed.
How to Flush a Fuel Injector
Clean fuel injectors help keep your engine running smoothly and efficiently. Clogged fuel injectors result in reduced fuel economy and loss of power.

Related questions

Fuel leak.
I would ask for help from a mechanic to look at your fuel system. They will be able to find the leak and make the necessary repairs so that your fuel no longer leaks (https://www.yourmechanic.com/services/fuel-is-leaking-from-gas-tank-inspection).
Gas not getting to the engine
An easy way to test for a failed fuel pump is to have a certified mechanic, like one from YourMechanic, spray a little bit of starting fluid into the vehicles intake and try to start it again. If this starts...
I had O2 sensor replaced twice in 4 months, but the engine light came on again 2011 Audi A6.
Hi there. If the check engine light is still on for the oxygen sensors, look at the description of the code to see if the sensor relates to a lean or rich condition. If the code does not, then there...

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