Skip the auto shop - Our mechanics make house calls

Q: Why Can’t I Top Off the Gas Tank?

asked by on

Why can’t I top off the gas tank?

A: The inability to top off your gas tank may ...

The inability to top off your gas tank may be the result of the car’s design or may be an indicator of an obstruction in the fuel fill neck of your car.

When you put fuel into your gas tank, the air inside the tank has to be vented off. This air usually escapes out through the fuel fill neck and back through the boot on the fill nozzle.

When the tank is nearly full, the air is no longer able to escape from the fuel tank. This leaves the air in the fuel fill neck that has to be vented back through the fill nozzle as you continue filling the tank.

The fuel fill neck has less capacity than the tank, so it fills up quickly. This creates an increase in air pressure inside the fill neck. This "back pressure" is sensed inside the fuel nozzle and automatically shuts off the fuel flow into your tank.

The purpose of shutting off the fuel flow into your tank is to prevent the possibility of damage to your vehicle’s evaporative emissions system. If you top off your fuel tank, you can create a situation where the charcoal canister in your car that is designed to store fuel vapors may become saturated with liquid fuel. A saturated canister will leak fuel onto the ground. If your charcoal canister becomes saturated, it must be replaced.

Another reason for not being able to top off your tank is actually described as a slow fill. Slow filling is caused by a mis-routed fill pipe or a kinked fill hose that restricts the flow of gasoline into the fuel tank. This can sometimes be resolved by re-routing the fuel fill pipe or hose. The fuel filler neck may even require replacement.

In order to reduce gas spillage and prevent liquid gasoline from entering the fuel vapor canister, fuel pumps sense when the fuel level in the tank is sufficiently high and halt the flow of fuel into the tank. Keep in mind, while it is possible to add gas to the tank after the pump initially stops pumping fuel, doing so could damage the fuel vapor canister and lead to an expensive repair. There is also a chance fuel could spill from the tank.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Does Gas Go Bad?

Gasoline, like milk, goes bad over time. Gasoline is usually good for about 6 weeks before it starts to turn into a varnish-like substance. This varnish-y fuel is sticky and makes all of your fuel system components non-operational. If...

Q: White smoke and a knocking sound after fixing the diesel pump problem

Hello, thanks for writing in about your Toyota. If you are getting white smoke and a knocking sound it is most likely coming from the engine fuel injection. It's either out of timing or flooding the engine with too much...

Q: The fuel gauge moves up and down I never know how much gas I have

The fuel sending unit is in the fuel tank mounted to the fuel pump assembly. The sending unit is shorted or open and needs to be tested using a gauge tester or by removing it and measuring the resistance of...

Related articles

How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
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...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.