Q: Why Can’t I Top Off the Gas Tank?

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.