The Positive Crankcase Ventilation (PCV) system takes blow-by and other vapors and draws them into the intake manifold under high vacuum signal conditions. They are mixed with the air/fuel mixture and burned to reduce HC emissions. This allows oil vapor to be introduced into the combustion process also alleviating crankcase pressure and reducing the possibility of oil leaks.
Hydrocarbon vapors present in blow-by gasses provide additional enrichment and may promote elevated levels of both hydrocarbon and carbon monoxide emissions. Lack of scheduled oil changes can allow sludge and moisture to build up in the PCV system and lead to blockage of the oil and PCV passages. Fuel contaminated oil can be verified by monitoring tailpipe emissions and temporarily disabling the PCV valve.
Improper PCV system operation can cause oil leaks, oil consumption, poor idle, stalling, increased crankcase pressures, and excessive wear. Increased crankcase pressure due to poor circulation will result in increased levels of blow-by gases and possibly an oil saturated air filter. If the PCV system is leaking, it can cause a vacuum leak or result in a leaner air-fuel mixture at idle. A blocked PCV system can cause a rich air-fuel mixture.