Valve - PCV: Operation

A typical enclosed crankcase ventilation system is shown in the graphic.

Fig 1: Positive Crankcase Ventilation System Schematic

Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.

The PCV system operates by engine intake manifold vacuum. Filtered air is routed into the crankcase through the air cleaner hose. The metered air, along with crankcase vapors, are drawn through the PCV valve (4) and into a passage in the intake manifold. The PCV system manages crankcase pressure and meters blow by gases to the intake system, reducing engine sludge formation.

The PCV valve contains a spring loaded plunger. This plunger meters the amount of crankcase vapors routed into the combustion chamber based on intake manifold vacuum.

Fig 2: Cutaway Of PCV Valve (Low Vacuum)

Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.

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

Fig 3: Cutaway Of PCV Valve (High Vacuum)

Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.

During periods of high manifold vacuum, such as idle or cruising speeds, vacuum is sufficient to completely compress spring. It will then pull the plunger to the top of the valve. In this position there is minimal vapor flow through the valve.

Fig 4: Cutaway Of PCV Valve (Moderate Vacuum)

Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.

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.