Positive Crankcase Ventilation

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Positive Crankcase Ventilation (PCV) system is used to remove crankcase blow-by gases from crankcase by using a PCV valve and manifold vacuum. Crankcase blow-by gases are drawn through the PCV valve and then returned back into the intake manifold with incoming air/fuel mixture. The PCV valve contains a spring loaded plunger. The plunger meters the amount of crankcase vapors routed into the combustion chamber based on intake manifold vacuum.

When the engine is not operating, or during an engine backfire, the spring forces the plunger back against the seat. See Fig 1 . This will prevent vapors from flowing through the valve.

During periods of high manifold vacuum, such as idle or cruising speeds, vacuum is sufficient to completely compress the spring. It will then pull the plunger to the top of the valve. See Fig 2 . In this position there is minimal vapor flow through the valve. During periods of moderate manifold vacuum, the plunger is only pulled part way back from inlet. This results in maximum vapor flow through the valve. See Fig 3 .

Fig 1: Engine Off Or Engine Backfire - No Vapor Flow (Typical PCV Valve)

Courtesy of DAIMLERCHRYSLER CORPORATION

Fig 2: High Intake Manifold Vacuum - Minimal Vapor Flow (Typical PCV Valve)

Courtesy of DAIMLERCHRYSLER CORPORATION

Fig 3: Moderate Intake Manifold Vacuum - Maximum Vapor Flow (Typical PCV Valve)

Courtesy of DAIMLERCHRYSLER CORPORATION