The gas cap consists of a plastic body with either threads (older caps) or lugs that locates into the filler neck of the fuel tank. Quality caps incorporate finely calibrated springs and valves. Caps may come in standard or locking types that minimize the chances of tampering with the fuel tank.
The gas cap used to simply cover the opening of the fuel tank filler neck, but eventually strong safety and emissions initiatives pushed cap designs to accomplish other tasks. The gas cap is now a major safety component that prevents the flow of fuel back out of the tank filler when a car is in an accident, including rollovers. This helps to reduce the chances of fuel leaks and their associated dangers. The gas cap is also an integral part of the onboard diagnostics system (OBDII) and emissions control system, which are interrelated.
The cap prevents the leakage of fuel vapours from the filler neck, which can be a major source of hydrocarbon emissions. A leaking or missing gas cap can result in the release of 90 litres of fuel per year into the atmosphere.
Always make sure your car's gas cap is reinstalled properly after each fill-up. If you lose the cap, replace it immediately for safety and emissions reasons. A loose gas cap can cause your car's SERVICE ENGINE SOON or CHECK ENGINE light to come on. If you find the cap loose, turn it to the right until it "snaps". It may take several start-and-run cycles for the light to reset. In areas with an emissions testing program, the gas cap may be tested for its ability to hold pressure. If the cap fails, it will need to be replaced. When you replace your car's gas cap, make sure it's made by a reliable manufacturer and that it's the right cap for that specific make and model. Beware of "one size fits all" caps and low-quality alternatives that may jeopardize your safety and your car's emissions control system.