DESCRIPTION
The crankcase ventilation system is designed to prevent contaminating hydrocarbons from escaping to the atmosphere. This is accomplished by routing the vapors from the crankcase through a vacuum-controlled ventilating valve (PCV Valve) into the intake manifold. There, they 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 the carburetor or on rocker arm cover.
When the engine is operating, fresh air enters the positive crankcase ventilation system through the air cleaner and filter.
The 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 70)
Crankcase Ventilation System (Typical) Air mixes with blow-by gases and air/fuel mixture, and burns in combustion chamber. Scheme 70
The ventilator valve is constructed so it is held closed by spring pressure when engine is not running. (Scheme 71) This prevents an accumulation of hydrocarbon fumes from collecting in the intake manifold, which could result in hard starting.
PCV Valve & Airflow (Typical) Air flows through valve when engine is running. Scheme 71
As the engine is started, manifold vacuum pulls the valve open against spring pressure. As long as there is engine vacuum, the valve floats, permitting crankcase fumes to enter the intake manifold.
A baffle in the rocker arm prevents oil from being drawn into the intake manifold through the ventilator valve.
In the event of an engine backfire through the intake manifold, the ventilator valve shuts, preventing any flow through it. This action prevents the ignition of fumes in the crankcase.
During certain engine operations, more blow-by is created than the ventilator valve can handle. The excess amount is returned to the air cleaner and carburetor through the rocker arm cover and breather assembly. It is then burned in the engine.
The breather assembly acts as a separator to keep oil from being drawn into the air cleaner during this operation.
MAINTENANCE
An engine may idle slow or rough due to a clogged ventilator valve or system. Therefore never adjust the carburetor idle without first checking the valve and system.
If the ventilator valve or system becomes clogged, all crankcase ventilation will stop, and serious engine damage could result.
Although the following manufacturers' service procedures give specific intervals, it is recommended the crankcase ventilation system be checked more frequently if vehicles are operated under severe conditions (extreme dust, prolonged idling, trailer hauling or short trips in cold weather).
PCV VALVE
Check every 15,000 miles and replace every 30,000 miles on Light Duty Emission models (up to 8500 lbs. GVW). Check every 12,000 miles and replace every 24,000 miles on Heavy Duty Emission models (over 8500 lbs. GVW). Valve is located on rocker cover.
FILTER ELEMENT
Replace every 30,000 miles on Light Duty Emission models; every 24,000 miles on Heavy Duty Emission models. Filter is located in carburetor.
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 the following checks
- Remove PCV valve from its mounting. If valve is functioning properly, a hissing noise will be heard as air passes through it. A 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.
- Re-install PCV valve, and then remove crankcase air inlet hose at air cleaner.
- Loosely hold a piece of stiff paper over opening at end of inlet hose. Paper should be sucked against hose opening with a noticeable force after sufficient time has elapsed for crankcase pressure to lower (usually about a minute). As a final check, stop engine, remove PCV valve and shake it, A metallic clicking noise should be heard, indicating valve is free.
- If system passes both the 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.