Evaporative Emissions System: Description and Operation

Fig.35 - Evaporative Emission Control System:

The evaporative emission control system, Fig. 35, is employed to prevent evaporative fuel in the fuel tank from being discharged into the atmosphere.

Components for this system vary according to model year. Systems through 1978 include a charcoal cannister, a check valve on high altitude and Calif. models, a two way valve on high altitude models, two orifices, and two reserve tanks on station wagons. 1979 systems consist of a charcoal cannister, check valve, two orifices, and two reserve tanks on station wagons. 1980 systems consist of a charcoal cannister, check valve, two way valve, and vapor separator. 1981-82 systems consist of a charcoal cannister, two way valve, purge control valve, thermal vacuum valve, solenoid valve, check valve, and vapor separator.

Fuel vapor evaporated from the fuel tank is routed into the canister located in the engine compartment. When the engine is idling, this vapor is stored in the canister because the purge valve on the canister is closed by the force of high intake manifold vacuum. During this time, the vapor is absorbed in the activated charcoal particles.

As the engine speed is increased, the purge valve opens because the vacuum in the carburetor above the throttle valve becomes as high as the vacuum in the intake manifold and the return spring overcomes the vacuum force to raise the purge valve. The absorbed vapor is drawn from the canister into the intake manifold through the open purge valve and purge hose. While the purge valve is open, fresh air is drawn into the canister through the bottom filter, purging the absorbed vapor from the activated charcoal.

When the engine is not running, the purge valve is opened by the return spring but the vapor remains in the canister because there is no vacuum to draw the vapor out of the canister at this time.