Why Can’t I Top Off My Tank?

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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.


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Pardeep

20 years of experience
648 reviews
Pardeep
20 years of experience
Subaru Outback - Brake Pads Replacement (Front, Rear) - San Jose, California
If I could give 6 stars out of 5 I would. Very friendly and accomodating. Please retain and take care of Pardeep.
Honda Odyssey - Brake Pads Replacement (Front, Rear) - San Jose, California
Great service. Timely, efficient, and friendly. Also gave good advice on services I actually needed.

Andrew

14 years of experience
236 reviews
Andrew
14 years of experience
Ford Escape - Brake Pads Replacement (Front) - Portland, Oregon
Andrew has come to fix my car on two occasions and both times he was on time for the appointment, professional , and extremely thorough in his examination of my car and diagnosis. He is honest and straightforward as to what my vehicle actually needs done immediately, and what I can hold off on for awhile. Money can be tight sometimes, so I do appreciate that. I highly recommend this mechanic!

Christopher

19 years of experience
152 reviews
Christopher
19 years of experience
Honda Pilot - Brake Pads Replacement (Rear) - Land O Lakes, Florida
Would book again. On time, knowledgeable and friendly. Will definitely have him on my next appointment.

Theodore

15 years of experience
1132 reviews
Theodore
15 years of experience
Subaru Outback - Brake Pads Replacement (Front, Rear) - Renton, Washington
We have had previous work done by Theodore. Really appreciate his thoroughness and professionalism!

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