Evaporative Emissions System: Description and Operation

These models use a manifold purge system. The charcoal canister media has been changed with a more efficient carbon, and a purge control valve situated atop the charcoal canister controls the evaporative system.
The purge signal (EGR port, spark port or manifold vacuum), actuates the purge valve to allow purging of the canister through the purge line. When the engine is off, the purge valve directs fuel vapors from the fuel tank and carburetor bowl to the canister. An exception to this would be when the engine compartment is below the temperature where sufficient gasoline vaporization occurs. During this time the thermal vent valve (on some models) closes, stopping flow in either direction in the bowl vent line. The purpose of the thermal bowl vent valve is to prevent fuel tank vapors from migrating up the bowl vent line and out the internal vent of the carburetor when the fuel bowl is not vaporizing. When the fuel bowl is vaporizing, the thermal valve is open, allowing flow to the canister. Also, the internal fuel bowl vent valve must be open (at idle) and/or the solenoid vent valve must be open (ignition off) to allow flow into the canister.