PCV Valve

Valve is operated by pressure difference between crankcase and intake manifold. When there is no pressure difference (engine not running) or when pressure of intake manifold is greater than that of crankcase, valve is closed.

During engine idle, high manifold vacuum overcomes valve spring and valve is pulled toward intake manifold side by vacuum. This causes air to pass through restricted passage between valve and housing. During normal engine operation, valve stays in a position where spring pressure and intake manifold vacuum balance. Amount of airflow then depends on position of valve.