gas cap
The cap that closes a gas tank and may be opened to refill it.
gas cap: sealed door to your fuel tank
A gas cap is the removable closure that seals the filler neck of a vehicle's fuel tank. Its primary job is to prevent fuel vapors from escaping, keep dirt and water out of the tank, and maintain the integrity of the fuel system's pressure. Most modern caps are threaded and twist onto the filler neck, though older vehicles sometimes used hinged or push-on designs.
The cap does more than sit passively. It houses a spring-loaded seal that creates an airtight closure, and many caps include a secondary vent valve that allows air to enter the tank as fuel is drawn out. Without this vent, the fuel pump would eventually create a partial vacuum inside the tank, starving the engine of fuel. Some caps also incorporate a float valve to prevent overflow during refueling.
Gas caps come in several functional types. The simplest are non-venting caps used on older vehicles with gravity-fed systems. Modern vented caps have a one-way valve that opens at a specific pressure differential, typically around 0.5 to 2 pounds per square inch. Capless fuel systems, found on many recent vehicles, eliminate the removable cap entirely and use an internal spring-loaded door in the filler neck instead.
Why caps fail and what happens
Fuel caps degrade through exposure to fuel vapors, temperature swings, and age. A cracked or missing cap allows fuel to evaporate and triggers the check engine light on modern vehicles, because the onboard diagnostics system detects abnormal vapor pressure. A damaged or incorrectly sealed cap can also admit water and contamination into the tank, leading to rust, sediment buildup, and fuel system corrosion.The term 'gas cap' is North American usage; British and European mechanics refer to it as a fuel cap or fuel filler cap. The cap's specification, including pressure rating and vent design, varies by vehicle model and emissions standard. Replacement caps are inexpensive, typically ten to thirty dollars, and are among the first things a technician checks when a vehicle reports fuel system diagnostic trouble codes.