Positive Crankcase Ventilation: Description and Operation

CRANKCASE VENTILATION SYSTEM DESCRIPTION

Crankcase Ventilation System Description:

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 PCV valve
- The crankcase vent (breather) or air inlet tube
- The air inlet filter or separator, if used
- Any hoses or couplers

OPERATION
The primary control of engine crankcase blow-by gas is the positive crankcase ventilation (PCV) valve. The PCV valve meters the flow of blow-by gas according to the manifold vacuum signal.

PCV VALVE

PCV Valve Operation:

One side of the PCV valve is exposed to manifold vacuum and the other side is exposed to crankcase gases. The PCV valve allows some vacuum pressure to pass through the valve orifice and create a low pressure condition in the crankcase. The blow-by gases in the crankcase are then drawn into the intake system and consumed in the normal combustion process. The volume of blow-by gas entering the intake manifold is precisely controlled In order to maintain idle quality. The correct PCV valve with the proper calibration must be used. The relationship between the flow of blow-by gas and the engine manifold vacuum is charted in the table.

RESULTS OF INCORRECT OPERATION
A plugged valve or hose may cause any of the following conditions:
- A rough engine idle
- Engine stalling or low engine idle speed
- High engine crankcase pressure
- Engine oil leaks
- Engine oil in the air cleaner
- Oil sludge in the engine
- Engine oil consumption
- Excessive exhaust emissions

A leaking valve or hose may cause any of the following conditions:
- A rough engine idle
- Engine stalling
- High engine idle speed
- Incorrect engine crankcase pressure
- Excessive exhaust emissions