Exhaust Emissions System - EVAP: Operation

The fuel vapor line, and on some models the filler neck, extends partially into fuel tank, causing an air pocket above fuel level in tank. As fuel expands, or when pressure in tank reaches 5 psi (.35 kg/cm² ), valve in two-way check valve opens and allows fuel vapor to travel to charcoal canister.

On 1974 models, high ambient temperatures will also cause fuel in carburetor float chamber to expand. When the engine is not running, the carburetor float chamber control valve is open to the carbon canister and fuel vapors from the float chamber pass to the canister where they are absorbed by activated charcoal.

The charcoal canister removes gasoline vapors before venting to atmosphere. As fuel level drops, other valve in two-way check valve opens and air is drawn into fuel tank. Check ball allows fuel returning to tank to flow from pump to tank only.