Positive Crankcase Ventilation (PCV Valve)
The (PCV) positive crankcase ventilation system removes blowby gases that bypass the piston rings during the power stroke. It consists of a valve and a supporting hose connected between the intake manifold and the cylinder valve cover.
When an engine idles, a high vacuum holds the valve tight against its seat. It prevents the valve from passing blowby gases into the intake.
A stuck open PCV valve results in a massive vacuum leak and a rough idle. A stuck closed or blocked valve causes excessive crankcase pressure and leaking seals. When the engine is accelerated, the manifold vacuum drops, allowing the valve to open. Blowby is ignited in the combustion process instead of released into the atmosphere.