Positive Crankcase Ventilation: Description and Operation

PCV System:

The [1][2]PCV (Positive Crankcase Ventilation) system is provided to prevent the blow-by gases from being emitted into the atmosphere. The term "blow-by gas" stands for combustion gas which has blown through the cylinder-to-piston clearance, and contains a large amount of unburned gases such as CO and HC.

PCV Valve:

The primary control is through the PCV valve which meters the flow at a rate depending on intake manifold vacuum. When the vacuum in the intake manifold is low (throttle valve open), the PCV valve is wide open due to its spring force. Thus, a large amount of the blow-by gas is drawn into the intake manifold. When the vacuum in the manifold is high, the PCV valve opening becomes regulated, due to high vacuum and the opposing spring force.

To maintain idle quality, the PCV valve restricts the flow when intake manifold vacuum is high. If abnormal operating conditions arise, the system is designed to allow excessive amounts of blow-by gases to back flow through the crankcase vent tube into the air cleaner to be consumed by normal combustion. Proper operation of the PCV system is dependent upon a sealed engine.