Skip the auto shop - Our mechanics make house calls
  1. Home
  2. Articles
  3. Why Can’t I Top Off My Tank?

Why Can’t I Top Off My Tank?

top off gas 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
Icon-warranty_badge-02

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

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

More related articles

P0222 OBD-II Trouble Code: Throttle/Pedal Position Switch/Sensor B Circuit Low Input
P0222 code definition Throttle/Pedal Position Switch/Sensor B Circuit Low Input What the P0222...
P0240 OBD-II Trouble Code: Turbocharger Boost Sensor B Circuit Range/Performance
P0240 code definition Turbocharger Boost Sensor B Circuit Range/Performance What the P0240 code means P0240 is an OBD-II generic code triggered when the Engine Control Module (ECM) detects the intake boost...
P2422 OBD-II Trouble Code: Evaporative Emissions Control System (EVAP) Vent Valve Stuck Closed
Diagnostic Trouble Code (DTC): P2422 P2422 code definition Evaporative Emissions Control System (EVAP)...


Related questions

Q: Gas tank seal from the filler neck hose.

The nozzle I believe your are mentioning is the anti-siphon and roll-over check valve. It does not require a perfect seal to the tank. The rubber filler hose slides over it, sealing in fuel vapors. I hope this helps. Put...

Q: 2001 Ford Focus Won't start

Hi there. The fuel pump is in the fuel tank. You will have a fuel test port on the engine fuel rail. You should have fuel pressure when you cycle the key to on for a few seconds and during...

Q: Filled in regular instead of premium fuel, CEL came on, P0022 code

Hello there, the P0022 code is indicating that the car's computer has over-retarded the engine timing. This makes sense with the 87 octane gasoline used instead of the 93 octane gasoline. The car was most likely attempting to adjust to...