The fuel cap on your car is something that you probably have never given much thought to. To most people it is little more than a plug that is taken off to fill the tank and then replaced until it is time to fill up again. All caps, both locking fuel caps, and non-locking versions are quite complex actually as they are required to meet specifications that a few years ago were never even thought of.
The actual fuel cap:
The actual cap is made from a plastic material; to locate the cap on the filler neck of the fuel tank the cap either has threads or lugs. Locking fuel caps are very popular as they prevent any pilferage, however; non-locking caps are often used, especially when the filler neck is located behind a locking access door.
The purpose of the fuel cap:
Years ago the sole purpose of a gas cap was to cover the tank opening. This is no longer the case, today a fuel cap not only covers the opening into the fuel tank, it is designed as a safety feature, and it is an integral part of a cars emission control system. In the event the car should roll over in an accident, an old style cap let fuel flow freely out of the tank, the new design caps prevents this from happening thus reducing the potential of fire and explosion.
Locking fuel caps are also an important part of the pollution control system and the onboard diagnostic system, both of which are interrelated. The cap design is such that fuel vapors cannot escape into the atmosphere. Leaking fuel vapors can equate to the loss of as much as 30 gallons of fuel annually.
In the event of a broken cap or one which has not been replaced properly, the on-board diagnostics will relay a message to a light on the dash indicating that a problem exists.