OBDII Automotive Articles - Positive Crankcase Ventilation

Positive Crankcase Ventilation

The Positive Crankcase Ventilation (PCV) system provides effective evacuation of crankcase vapors. Fresh air from the air filter housing is supplied to the crankcase, where it is mixed with the blow-by gasses and passed through the PCV valve and into the intake manifold. This mixture is then passed into the combustion chamber and burned.

The PCV valve provides primary control in this system by metering the flow of the blow-by vapors. When manifold vacuum is high at idle, the PCV valve restricts the flow to maintain the smooth idle.

Under the condition in which abnormal amounts of blow-by gasses are produced (such as worn cylinders or rings) system is designed to allow excess gasses to flow back through crankcase vent hose into the air inlet.

Spring pressure holds PCV valve closed when the engine is not running. This prevents hydrocarbon fumes from collecting in the intake manifold, a condition which could result in hard starting.

During engine operation, the manifold vacuum pulls the valve closed against spring pressure. As vacuum decreases with increased engine load (ROM), spring pressure begins to overpower vacuum strength. This allows PCV valve to open proportional to engine load and evacuation requirements. Should the engine backfire, the PCV valve closes to prevent ignition of fumes in the crankcase.