Fuel Filler Cap: Description and Operation

The pressure-vacuum filler cap is designed to hold a slight pressure on the fuel. Since the evaporation rate decreases under slightly higher pressures, the cap helps to reduce evaporative emissions.

Fuel Tank Pressure/Vacuum Filler Cap:

A rubber seal around the inside lip of the cap prevents fuel vapor from escaping to the atmosphere, as well as prevent fuel from spilling in the event of vehicle roll-over.

When vacuum in the tank exceeds a pressure differential of 0.5 psi to atmospheric pressure, the vacuum relief valve will allow atmospheric pressure to enter the tank.

If fuel tank pressure becomes 1.8 psi above atmospheric pressure, the pressure relief portion of the cap will open to the atmosphere.