Fuel Filler Cap

Although all of us know how to use the fuel filler cap, it is actually more complicated than it looks. Inside the fuel filler cap is a pressure release valve. This allows it to vent the fumes in the gas tank if they build up to predetermined levels. Until the fumes reach these levels, they are shunted through the charcoal canister which collects the fuel from the air before the air escapes. When the fumes build up above the predetermined (differing from car to car) threshold level of the fuel filler cap's pressure release valve, they are vented into the atmosphere. The fuel filler cap has a rubber flange around the neck. This flange should be inspected for cracks or inflexibility. If the flange does become cracked or inflexible, it should be replaced to keep the environment clean.