DESCRIPTION & OPERATION
Crankcase ventilation systems prevent hydrocarbons from entering the atmosphere. This is accomplished by drawing vapors from the crankcase through a Positive Crankcase Ventilation (PCV) valve and routing them into the combustion chamber via the intake manifold. In the combustion chamber these vapors are burned in the combustion process. (Scheme 172)
Scheme 172
The PCV valve is held closed by spring pressure when engine is not running. This prevents hydrocarbon fumes from collecting in the intake manifold, a condition which could result in hard starting.
During engine operation, manifold vacuum pulls the valve open against spring pressure, permitting crankcase fumes to enter the intake manifold. Should the engine backfire, the PCV valve will close to prevent ignition of fumes in crankcase.
During certain engine operations, more blow-by is created than the PCV valve can handle. Excess blow-by is directed to the air cleaner and carburetor or Throttle Body through a breather assembly in the valve cover. It is then burned in the combustion chamber. The breather assembly acts as a separator to keep oil from being drawn into air cleaner during this operation.
MAINTENANCE
An engine may idle slow or rough due to clogged PCV valve or ventilation system. Never adjust fuel injection system without first checking valve and system. If PCV valve or system is clogged, all crankcase ventilation will stop and could result in serious engine damage.
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 valve cover.
FILTER ELEMENT
Replace filter element every 30,000 miles on Light Duty Emission models and every 24,000 miles on Heavy Duty Emission models. Filter is located in air cleaner housing.
COMPONENT TESTING
- Start engine and allow it to reach normal operating temperature. Ensure engine is idling at normal curb idle. Remove PCV valve from grommet. If valve is okay, hissing will be heard as air passes through it. Strong vacuum should be felt over valve inlet.
- With finger over valve inlet, check for vacuum leaks at all connections. Reinstall PCV valve and remove crankcase air inlet hose at air cleaner.
- Loosely hold piece of stiff paper over opening at end of inlet hose. After about a minute, paper should be sucked against hose opening with noticeable force. Remove and shake PCV valve. Ensure a metallic clicking noise can be heard, indicating valve is free.
- If system passes both engine running and stopped tests, it is functioning properly. If it has failed either test, replace defective components and retest. If it does not pass on second try, clean system.