Positive Crankcase Ventilation: Description and Operation

PCV System Operation:

DESCRIPTION
The compressed combustion gas which passes by the piston into the crankcase is known as blowby gas. This gas contains large amounts of CO and HC. The Positive Crankcase Ventilation (PCV) system is provided to prevent the blowby gas from being emitted into the atmosphere, and it operates as follows.

^ 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.
^ 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.

OPERATION
The primary control is through the PCV valve which meters the flow at a rate depending on manifold vacuum.

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 blowby gases to flow back through the crankcase vent tube into the air cleaner to be consumed by normal combustion.

RESULTS OF INCORRECT OPERATION
A plugged valve or hose may cause:

^ Rough idle.
^ Stalling or slow idle speed.
^ Oil leaks.
^ Oil in air cleaner.
^ Sludge in engine.

A leaking valve or hose would cause:

^ Rough idle
^ Stalling.
^ High idle speed.

IMPORTANT
The correct PCV valve for the engine and equipment type is very important. Use of wrong or substandard parts may result in any of the problems just listed as well as excessive exhaust emissions.