Positive Crankcase Ventilation System (PCV)

The Positive Crankcase Ventilation System (PCV) flows fresh air into the engine crankcase to dilute the unburned hydrocarbons present and route them into the intake manifold where they are consumed during the combustion process.

The PCV valve controls or meters this flow of crankcase vapors during all modes of engine operation. Each PCV valve is sized for a specific application to provide a flow rate that will meet the engines needs during idle, high speed, and heavy load conditions. PCV valves are not interchangeable based on their external dimensions; they must meet the designed flow requirements.

The PCV valve meters the vapor flow under all engine operating conditions.

During Deceleration, Low Speed, and Idle
During engine deceleration and idle, manifold vacuum is high. The high vacuum overcomes the force of the PCV valve spring and the valve bottoms in the manifold end of the valve housing. This does not completely stop the flow of crankcase vapors into the intake manifold, but it does restrict them to a specified value for each engine.

During Acceleration, High Speed, or Heavy Load
When the engine is accelerated, operated at high speed, or under heavy load, the manifold vacuum is less than it would be at idle or during deceleration. During acceleration, the PCV valve spring force is stronger than the vacuum pull from the intake manifold. This forces the valve toward the crankcase end of the valve housing. With the valve in this position, more crankcase vapors can flow into the intake manifold because the flow path offers less restriction. This larger flow path offsets the fact that the manifold vacuum is less during these operating conditions.