DESCRIPTION
Crankcase ventilation systems are designed to prevent contaminating hydrocarbons from escaping to the atmosphere. This is accomplished by routing vapors from the crankcase through a vacuum-controlled ventilating valve (PCV Valve) into the intake manifold. In the intake manifold, the crankcase vapors mix with the air/fuel mixture and are burned in the combustion process.
OPERATION
Air is supplied to the crankcase ventilation system through a crankcase ventilating filter assembly, located in air cleaner housing or on rocker arm cover.
When engine is operating, fresh air enters crankcase ventilation system through the air cleaner and filter.
Air then flows into the rocker arm cover and valve compartment. It combines with blow-by gas and unburned air/fuel mixture and burns in combustion chamber. (Scheme 63)
Crankcase Ventilation System (Typical). Scheme 63
Ventilator valve is held closed by spring pressure when engine is not running. (Scheme 64) This prevents accumulation of hydrocarbon fumes from collecting in intake manifold, which could result in hard starting.
Scheme 64
When engine is started, manifold vacuum pulls valve open against spring pressure. As long as there is engine vacuum, the valve floats, permitting crankcase fumes to enter intake manifold.
A baffle in rocker arm cover prevents oil from being drawn into intake manifold through ventilator valve.
If the engine backfires, the ventilator valve will close. This will prevent ignition of fumes in crankcase.
During certain engine operations, more blow-by is created than ventilator valve can handle. Excess blow-by is returned to air cleaner and carburetor through rocker arm cover and breather assembly. It is then burned in the combustion chamber.
A breather assembly acts as separator to keep oil from being drawn into air cleaner during this operation.
MAINTENANCE
Engine may idle slow or rough due to clogged ventilator valve or system. Therefore, never adjust carburetor idle without first checking valve and system.
If ventilator valve or system becomes clogged, all crankcase ventilation will stop, and serious engine damage could result.
Although following manufacturers' service procedures give specific intervals, it is recommended the crankcase ventilation system be checked more frequently if vehicle is operated under severe conditions (extreme dust, prolonged idling, trailer hauling or short trips in cold weather).
PCV VALVE
Check PCV valve every 15,000 miles and replace it every 30,000 miles on Light Duty Emission models (up to 8500 lbs. GVW). Check PCV valve every 12,000 miles and replace it every 24,000 miles on Heavy Duty Emission models (over 8500 lbs. GVW). Valve is located on rocker arm cover.
FILTER ELEMENT
Replace filter element every 30,000 miles on Light Duty Emission models; every 24,000 miles on Heavy Duty Emission models. Filter is located in air cleaner housing.
SYSTEM TESTING
To test crankcase ventilation system, start engine and allow it to reach normal operating temperature. Make sure engine is idling at normal curb idle, and perform following checks
- Remove PCV valve from its mounting. If valve is functioning properly, hissing noise will be heard as air passes through it. Strong vacuum should be felt when your finger is placed over valve inlet.
- While finger is over inlet, check for presence of vacuum leaks in hose line and at all connections. Reinstall PCV valve, remove crankcase air inlet hose at air cleaner.
- Loosely hold piece of stiff paper over opening at end of inlet hose. Paper should be sucked against hose opening with noticeable force after sufficient time has elapsed for crankcase pressure to lower (usually about a minute). For final check, stop engine, remove PCV valve and shake it. Metallic clicking noise should be heard, indicating valve is free.
- If system passes both engine running and stopped tests, it is functioning properly. No further tests are required. If it has failed either test, replace appropriate components and retest. If it does not pass on second try, clean system.