Evaporative Check Valve: Description and Operation

When the engine is not operating, fuel vapor from the fuel tank reaches the tank pressure control valve. This valve is held closed by a diaphragm spring. Fuel vapor then flows through a bypass orifice to be stored in the charcoal canister. Should a negative pressure develop within the fuel tank, ambient air will flow through the charcoal canister and the bypass orifice into the fuel tank. With engine operating, manifold vacuum forces the tank pressure control valve open. Fuel vapor is then stored in the charcoal canister.