Positive Crankcase Ventilation: Description and Operation

Fig. 7 PCV Valve:

Intake manifold vacuum removes crankcase vapors and piston blow-by from the separator. Emissions pass through the PCV valve into the intake manifold plenum. The vapors become part of the calibrated air-fuel mixture, are burned and then expelled with the exhaust gases. The air cleaner supplies make up air when the engine does not have enough vapor or blow-by gases.

PCV Valve
The Positive Crankcase Ventilation (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.

Fig. 9 Engine Off Or Engine Backfire--No Vapor Flow:

When the engine is not operating or during an engine backfire, the spring forces the plunger back against the seat. This prevents vapors from flowing through the valve (Fig. 9).

Fig. 10 High Intake Manifold Vacuum--Minimal Vapor Flow:

When the engine is at idle or cruising, high manifold vacuum is present. At these times manifold vacuum is able to completely compress the spring and pull the plunger to the top of the valve (Fig. 10). In this position there is minimal vapor flow through the valve.

Fig. 11 Moderate Intake Manifold Vacuum--Maximum Vapor Flow:

During periods of moderate intake manifold vacuum the plunger is only pulled part way back from the inlet. This results in maximum vapor flow through the valve (Fig. 11).