The positive crankcase ventilation system returns crankcase gases to the upper part of the intake manifold.
To remove crankcase gases to the upper part of the intake manifold, a positive crankcase ventilation (PCV) valve is provided. When the engine is running with the throttle valve partially open, crankcase gases are sucked into the upper part of the intake manifold through the PCV valve. As a rule, the valve capacity is sufficient to remove crankcase gases and a small amount of ventilation air. The ventilation air is then sucked into the crankcase through the hose connecting the air filter to the valve cover. When the engine is running with the throttle valve fully open, the vacuum in the upper part of the manifold is not enough to suck in crankcase gases through the valve and the flow of gases goes through the hose connection in the opposite direction.
On vehicles with excessive amounts of crankcase gases, the valve will not be able to remove them completely because some of the flow will go through the hose connection to the air filter under all operating conditions.
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