Too much oil is entering the combustion chamber. This is often caused by piston rings or cylinder walls that are badly worn. Oil may also be pulled into the chamber because of excessive clearance in the valve stem guides. If the PCV valve is plugged or inoperative it can cause a build-up of crankcase pressure which can force oil and oil vapours past the rings and valve guides into the combustion chamber.
More information On Basic Car Maintenance CD Rom
Information supplied by Champion Sparkplugs ©2009