When Engine is Running

When vacuum develops in the fuel tank or the engine crankcase and the difference between the relief side and the fuel tank or crankcase is 0.2 - 0.6 in. Hg, the relief valve opens and allows air from the air filter to enter the fuel tank or crankcase. This air replaces fuel vapors and brings the fuel tank or crankcase back to balanced atmospheric pressure.

On California models, the vent switching valve is closed to prevent air from flowing through the canister and into the carburetor. When vacuum exceeds a specified level, the canister purge valve is opened and air is drawn through the canister into the intake manifold.