Canister Control Valve (CCV) (High Altitude 4.3L Except "S" & "T" Series & V8)

CCV is vacuum operated. When the engine is not running, vapor from the fuel tank is stored in the carbon canister. When the vehicle is started, vacuum to the upper port draws in the internal vacuum diaphragm, opening the port between the canister and purge valve. When engine is off, internal spring pressure closes valve diaphragm, preventing vapor from venting to atmosphere.

The canister control valve acts as both vapor vent valve and purge valve. When engine is running, manifold vacuum from PCV system pulls lower diaphragm upward. When engine is operating above idle speed, control vacuum pulls upper diaphragm upward. This allows purging of canister through PCV system.