Fuel Cap

A cap that is missing, loose or has a worn seal lets fuel vapour escape. On cars with OBD-II leak testing, this is one of the most common reasons for an EVAP leak code and an engine warning light. Many newer cars have capless fillers with a spring-loaded flap instead. They can also leak if dirt holds the flap open.

Many modern cars have a capless filler with a spring-loaded flap instead of a screw cap. Diesel cars have caps too, but their tanks are not part of the same strict evaporative leak test.

The cap seals with a rubber gasket and clicks to show it is tight. Some caps include a valve that lets air in as fuel is used. Most screw caps have a ratchet that clicks when the cap is tight, and a valve that lets air in as fuel is used, so the tank does not collapse under vacuum.

After refuelling, tighten the cap until it clicks. If the warning light appears after a fill-up, check the cap first; the light usually goes out by itself after several drive cycles once the leak is gone. After refuelling, tighten the cap until it clicks. If a large EVAP leak code appears soon after refuelling, check the cap seal for cracks before looking for more expensive faults.