Fuel Tank: Description and Operation

Fig. 37 Evaporative emission system. (Typical):

Refer to Fig. 37 for a typical evaporative emission system. Complete system schematics are located in ``Vacuum Hose Routings.''




This system reduces hydrocarbon emissions from the fuel system by the use of activated charcoal in the carbon canister. With engine off, fuel vapor from the fuel tank is led into the carbon canister for storage.
The canister retains the vapor until it is purged by air drawn through the purge line to the intake manifold when the engine is running. With engine at idle, the purge control valve is closed but a small amount of air flows into the intake manifold through the constant purge orifice. As the engine speed increases and the ported vacuum rises, the purge control valve opens and the vapor is drawn into the intake manifold through the fixed orifice and the constant purge orifice. On high altitude except Calif. models, the vapor in the carburetor float chamber is led into the canister through the outer vent line because the vent switching valve is normally open.