Fuel Filler Cap: Description and Operation

Fuel Filler Cap:

PURPOSE
The Fuel Filler Cap has a vacuum relief valve built into it. The vacuum relief valve is designed to prevent fuel vapor from escaping into the atmosphere and to allow the introduction of air into the tank. This prevents the fuel tank from collapsing.

OPERATION
In order to prevent fuel tank collapse and not allow fuel vapor into the atmosphere, the Vacuum Relief Valve acts as a one-way valve or check-valve. This keeps constant atmospheric pressure in the fuel tank while not letting fuel vapors out.