PCV System: Operation
The crankcase ventilation system operates on manifold vacuum. Air is drawn from the crankcase, through the PCV valve and hose into the intake manifold becoming part of the calibrated air/fuel mixture. When the engine is not running, the PCV valve is held closed by spring pressure. This prevents hydrocarbon fumes from collecting in the intake manifold.
During engine operation, manifold vacuum pulls the valve open, against spring pressure, permitting crankcase fumes to enter the throttle body passage. Should the engine backfire, the PCV valve will close to prevent ignition of fumes in the crankcase.