Fuel Filler Cap: Description and Operation

Fuel Filler Cap Operation:

PURPOSE/OPERATION
The pressure-vacuum filler cap is designed to maintain a slight pressure in the fuel tank. This pressure occurs from normal heat expansion. By maintaining this pressure, fuel evaporation is reduced, this helps to reduce evaporative emissions. When vacuum in the tank exceeds specifications, the vacuum relief portion of the filler cap allows atmospheric pressure to enter the tank. If the pressure within the tank exceeds a specified value, the pressure relief valve opens to relieve excess pressure. This occurs when the pressure relief check valve within the fuel vapor line is plugged.