DESCRIPTION
The positive crankcase ventilation system is designed to prevent contaminating hydrocarbons, created in the crankcase, from escaping to the atmosphere.
This is accomplished by routing the vapors from the crankcase through a vacuum controlled ventilating valve (PCV) into the intake manifold. When the vapors reach the intake manifold they are mixed with the air/fuel mixture and are burned in the combustion process.
OPERATION
When the engine is operating, fresh air enters the positive crankcase ventilation system through the air cleaner assembly. Fresh air then flows through the crankcase breather and into the rocker arm/valve compartment.
The entering fresh air then combines with blow-by gases and unburned air/fuel mixture present in the crankcase. The combined gases are then drawn into the carburetor, through the PCV valve, by manifold vacuum. The combined gases mix with the air/fuel mixture and are burned in the combustion chamber. (Scheme 57)
Positive Crankcase Ventilation System (Typical). Scheme 57
The PCV valve is held closed by spring pressure when the engine is not running. This prevents hydrocarbon fumes from collecting in the intake manifold, resulting in hard starting. (Scheme 58)
Cutaway View of PCV Valve (Typical). Scheme 58
When the engine is started, manifold vacuum pulls the PCV valve open against spring pressure. As long as there is engine vacuum, crankcase fumes are allowed to enter the intake manifold.
A baffle in the rocker arm cover prevents engine oil from being drawn into the intake manifold through the PCV valve.
If the engine backfires through the intake manifold, the PCV valve closes and prevents any flow of gases through it. This is to prevent the ignition of fumes in the crankcase.
SERVICE PROCEDURES
An engine may idle slow or rough due to a clogged PCV valve or system. Never adjust idle speeds without first checking the entire PCV system.
| CAUTION | If a PCV system component becomes clogged, all crankcase ventilation will stop and serious engine damage could occur. |
Although the following manufacturers' service procedures give specific service intervals, it is recommended that the PCV system be serviced more frequently if vehicle is operated under severe conditions (extreme dust, prolonged idling, trailer hauling or short trips in cold weather).
PCV VALVE
Remove PCV valve from intake manifold or rocker arm cover and replace every 30,000 miles. DO NOT attempt to clean valve.
FILTER ELEMENT
Filter element should be replaced every 30,000 miles. On vehicles equipped with diesel engines, filter element should be cleaned every 15,000 miles.
SYSTEM TESTING
- Start engine and warm to normal operating temperature. Engine should be running at curb idle speed. Remove PCV valve from its mounting.
- If valve is functioning properly, a hissing noise will be heard as air is drawn through valve. Place finger over PCV valve inlet. A strong vacuum should be felt.
- If no vacuum is felt at PCV valve, check for blockage or restriction in PCV valve hose. If hose is okay, replace PCV valve. While finger is over inlet of valve, check for vacuum leaks in hose and at all connections.
- Turn engine off. Remove PCV valve from hose. Shake PCV valve. A clicking noise should be heard when shaking valve, indicating valve is free. If no clicking noise is heard, replace PCV valve.