Most 1971 and newer vehicles and some older models are equipped with an evaporative emissions system to collect and store gasoline vapors and gasoline from the carburetor and fuel system during engine operation or shut down. The vapors are collected and stored in a canister containing a charcoal filter and gasoline is returned to the fuel tank. The vapors are purged from the canister and drawn into the engine when the engine is started. A pressure vacuum gas cap is essential to the proper operation of this system.