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 PCV system routes the crankcase blow-by gas back into the intake system where the blow-by gas becomes part of the combustion process. The PCV system consists of the following components:

The primary control of engine crankcase blow-by gas is the oil separator. The oil separator separates the oil from the blow-by gases and meters the flow of blow-by gas according to the manifold vacuum signal. The manifold vacuum draws the blow-by gases from the oil separator into the valve cover then into the intake where it is consumed by the normal combustion process. The volume of blow-by gas entering the intake manifold is precisely controlled in order to maintain idle quality.

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

A faulty separator or hose may cause any of the following conditions: