System Components

TANK PRESSURE CONTROL VALVE
When the engine is not operating, fuel vapor from the fuel tank reaches the tank pressure control valve. This valve is held closed by a diaphragm spring. Fuel vapor then flows through a bypass orifice to be stored in the charcoal canister. Should a negative pressure develop within the fuel tank, ambient air will flow through the charcoal canister and the bypass orifice into the fuel tank. With engine operating, manifold vacuum forces the tank pressure control valve open. Fuel vapor is then stored in the charcoal canister.

VENT SWITCHING VALVE
The vent switching valve is an electrically operated solenoid valve built into the carburetor. When the engine is not operating, the valve opens to allow fuel vapor to enter charcoal canister. With engine operating, the valve closes, blocking ambient air flow from the charcoal canister to the carburetor.

CHARCOAL CANISTER
The charcoal canister is filled with activated carbon. This carbon stores fuel vapors emitted from the carburetor and fuel tank.
The canister incorporates a purge control valve which purges fuel vapor from the activated carbon so that it can be delivered to the intake manifold. The purge control valve diaphragm is activated by a carburetor timed vacuum signal. When the engine is not operating, the diaphragm spring holds the purge control valve closed. With the engine operating, carburetor ported vacuum is applied to the diaphragm. If vacuum exceeds a predetermined value, the diaphragm opens the purge control valve. Fuel vapor from the activated carbon is then delivered to the engine intake manifold. Fuel vapor purge rate is controlled by engine intake manifold vacuum and the purge control valve orifice.

THERMAL VACUUM VALVE
The thermal vacuum valve is connected in series with the purge signal line. It prevents vapor purge during engine warm-up under very cold driving conditions.