The compressed combustion gas which escapes past the piston rings into the crankcase is known as blow-by gas. Blow-by gas contains large amounts of CO and HC. The Positive Crankcase Ventilation (PCV) system prevents the blow-by gas from being emitted into the atmosphere. The Positive Crankcase Ventilation (PCV) system consists of the PCV valve, the air intake (breather) hoses or couplers and the air inlet filter or separator (If used).

The primary control of engine crankcase blow-by gas is through the PCV valve which meters the flow at a rate depending on manifold vacuum. The Positive Crankcase Ventilation (PCV) system operates as follows:

To maintain idle quality, the PCV valve restricts the flow of blow-by gas whenever intake manifold vacuum is high. When abnormally high crankcase (blow-by gas) pressure occurs, the system is designed to allow the excess quantity of blow-by gases to flow back through the crankcase vent tube into the air cleaner to be consumed by normal combustion.

A plugged valve or hose may cause any of the following conditions:

A leaking valve or hose may cause any of the following conditions: