Evaporative Emissions System: Description and Operation

Fig.1 - Evaporative Emission Control System:

All 1971-82 models have an evaporative emissions control system to control hydrocarbon emissions into the atmosphere from the fuel tank, lines, and carburetor. The system, Fig. 1, consists of a sealed fuel filler cap, check valves between the fuel tank and charcoal canister, a charcoal canister with a vacuum controlled purge line, fuel pump and lines with a check valve in the return line, and the float bowl on carburetor models.

Vapors accumulated in the fuel tank are vented through the one way check valve to the charcoal canister when pressure in the tank rises due to temperature changes. When temperature decreases, a vacuum is created in the tank, and the two way check valve opens venting the tank through the charcoal canister.

Fuel vapors are stored in the charcoal canister. When the engine is operating, with the throttle plate open, vacuum is applied to the purge valve on the charcoal canister. Fuel vapors stored in the canister are drawn into the intake manifold, and on some models, the air cleaner, and burned in the engine.

Fig.2 - Carburetor Bowl Vent Electrovalve:

When the engine is operating at idle, the carburetor float bowl on some models is vented through the charcoal canister. On 1980-82 models, an electromagnetic vent valve, Fig. 2, is used to control carburetor bowl venting. The valve is controlled by a relay and a switch on the idle stop. At idle, the valve is not energized, and vapors are vented to the canister through an orifice (H), Fig. 2. During acceleration, the valve is energized, and the passage to the canister is closed. The carburetor bowl is open to the atmosphere through passages (F) and (G), Fig. 2.