Positive Crankcase Ventilation: Description and Operation

Positive Crankcase Ventilation (PCV) System:

POSITIVE CRANKCASE VENTILATION SYSTEM

GENERAL DESCRIPTION
Blowby Gas
"Blowby gas" is the compressed gas and exploded gas which passes by the piston into the crankcase. Blowby gas contains a large amount of unburned HC gases and partially burned gases such as CO. The Positive Crankcase Ventilation ([1][2]PCV) system is provided to prevent the blowby gas from being emitted into the atmosphere.

Metered Orifice Type
The PCV system is a metered orifice type which supplies vacuum from the intake plenum through a vacuum hose to a metered orifice in the valve cover. The PCV system draws out blow-by gases at a controlled rate.

SYSTEM COMPONENTS
The PCV system consists of a [1][2]PCV hose and metered orifice mounted in a valve cover.

OPERATION
The PCV system draws out blow-by gas through the metered orifice, through the [1][2]PCV hose and into the intake plenum and is consumed by normal combustion.

Low 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 blowby gas is drawn into the intake manifold.

High Vacuum
On the other hand, when the vacuum in the manifold is high, the PCV valve opening is limited due to the high vacuum. Thus the amount of the blowby gas drawn into the intake manifold is small.

RESULTS OF INCORRECT OPERATION
A plugged metered orifice or hose may cause:
^ rough idle
^ stalling or slow idle speed
^ oil leaks
^ oil in air cleaner
^ sludge in engine

A leaking metered orifice or hose would cause:
^ rough idle
^ stalling
^ high idle speed