Fuel Filler Cap: Description and Operation

The fuel filler cap has both a vacuum valve and a pressure valve. If a negative pressure develops in the fuel tank, the ambient valve of the fuel filler cap will open. Ambient air will flow into the fuel tank through the vacuum valve.

When the development of a positive pressure in the fuel tank exceeds a specific value, the pressure valve will open and the fuel vapor will escape into the atmosphere. This valve does not function under normal operating conditions.