Evaporative Emission Control System Operation

Fig. 20 Evaporative emission control system. Exc. 4 cylinder engine:

6 CYLINDER & V8 ENGINES
In this system, Fig. 20, when the fuel tank is filled to the base of the filler tube, vapors can no longer escape, they become trapped above the fuel. Vapor flow through the vent line is blocked by the limiting valve, and the filler tube is blocked by fuel, preventing more fuel from entering the tank. At any time pressures in the tank rise above operating temperatures of the limiting valve, the valve opens and allows vapors to flow forward to the charcoal canister. Due to the configuration of the fuel tank on some models and all station wagons, vapor separator tanks are not required. The charcoal canister is a feature on all models for the storage of fuel vapors from the fuel tank and carburetor bowl. A vacuum port located in the base of the carburetor governs vapor flow to the engine. On some models, each corner of the fuel tank is vented and each of the hoses from these vents is connected to a vapor separator. A tube from the separator leads to the charcoal canister. Evaporated fuel vapor from the fuel tank flows through the separator to the canister. The canister used has three hoses and no purge valve. The purge valve previously located on top of the canister has been eliminated by using an additional ported vacuum connection on the carburetor for purging the canister. This utilizes the throttle plates of the carburetor as a purge valve. This system will improve hot idle quality by eliminating canister purging during idle. Some limited production, high performance vehicles will continue to use the earlier type two stage canister which utilizes an integral purge valve. This canister can be identified by four hose connections while the new type canister uses only three.