PCV Valve (Positive Crankcase Ventilation Valve)

It is part of the crankcase ventilation system. Some engines use an oil separator instead. It prevents pressure build-up. Many modern engines integrate the PCV function in the valve cover, which must be replaced as a unit. Excess crankcase pressure can be checked by removing the oil filler cap with the engine idling.

Engine vacuum draws gases through the valve.

A stuck valve can cause oil leaks or idle problems. A torn diaphragm can cause a whistling noise.