Positive Crankcase Ventilation: Description and Operation

Crankcase Ventilation Valve Flow:

DESCRIPTION
A crankcase ventilation system is used to consume crankcase vapors in the combustion process instead of venting them to the atmosphere. Fresh air from the air cleaner is supplied to the crankcase, mixed with blow-by gases and then passed through a crankcase ventilation valve into the intake manifold.

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

To maintain idle quality, this 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 inlet duct to be consumed by normal combustion.

RESULTS OF INCORRECT OPERATION
A plugged valve may cause:
- Rough idle.
- Stalling or slow idle speed.
- Oil leaks.
- Sludge in engine.

A leaking valve would cause:
- Rough idle.
- Stalling.
- High idle speed.