This system manages fuel vapor escape from the vehicle's fuel tank. It includes a charcoal canister, a purge control valve, and a special fuel filler cap with a vacuum relief valve. When the engine is off, fuel vapor accumulates in the tank and is stored in the charcoal canister. Upon engine start, air carries the vapor from the canister to be burned during combustion. As fuel is used, a valve opens to prevent a vacuum. The valve, canister, and purge control valve require regular service. US cars' filler caps have a vacuum relief valve to maintain tank pressure.