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 vapor in the carburetor float chamber is also led into the canister through the outer vent pipe because the vent switching valve is normally open.
With engine running, the vapor is retained in the canister until it is moved by air drawn through the purge line to the intake manifold. As engine speed increases, the ported vacuum rises and the purge control valve opens the orifice, allowing the vapor to travel through the purge line to the intake manifold. When coolant temperature is below 140°F (1985-86 Calif. models, 122°F), the purge control valve is closed by the thermal vacuum valve, preventing vapor from flowing into the intake manifold.




This system reduces hydrocarbon emissions from the fuel system by the use of activated charcoal in the carbon canister. With the engine off, fuel vapor from the fuel tank is led into the carbon for storage.
The vapor is retained in the canister until it is moved by air drawn through the bottom of the canister to the intake manifold with the engine running. The purge control valve is closed when the engine is running at idle.
A small portion of the stored vapor flows through the constant purge orifice into the intake manifold. As engine speed increases and throttle vacuum rises, the purge control valve opens and vapor is drawn into the manifold through both the main and constant purge orifices.