With these systems, any blow-by in excess of the system capacity (from a badly worn engine, sustained heavy load, etc.) is exhausted into the air cleaner and is drawn into the engine.

Proper operation of the crankcase ventilation system depends upon a sealed engine. If oil slugging or dilution is noted and the crankcase ventilation system is functioning properly, check the engine for a possible cause. Correct any problems.

If an engine is idling rough, check for a clogged crankcase ventilation valve, a dirty vent filter, air cleaner element, or plugged hose. Replace as required. Use the following procedure:

1. Remove the crankcase ventilation valve from the rocker arm cover.
2. Operate the engine at idle.
3. Place your thumb over the end of the valve in order to check for a vacuum. If there is no vacuum at the valve, check for the following items:

•	Plugged hoses.

•	The manifold port.

•	The crankcase ventilation valve.

4. Turn off the engine. Remove the crankcase ventilation valve. Shake the valve. Listen for the rattle of the check needle inside of the valve. If valve does not rattle, replace the valve.