Fuel Evaporation System: Operation
When ignition key is off, fuel tank and carburetor float bowl vapors are vented into charcoal canister. Carburetor and float bowl vapors are absorbed by carbon element in air cleaner on Pickup.
When engine is running above idle and warmed to normal operating temperature, thermo valve will open and allow vacuum to open purge control valve. Canister vapors are then drawn into intake manifold through purge control valve for burning. Purge control valve is kept closed during idle and engine warm up to reduce HC and CO emissions.
On turbo engines, purge control valve opens when the pressure difference between the turbo-charged and throttle-port pressures exceeds a pre-set value.