|MARK’S CORNER – AUTO REPAIR HELP|
POSITIVE CRANKCASE VENTILATION (PCV)
The PCV system removes vapors from the crankcase and delivers them to the intake manifold, to be burned in the combustion chamber. The system consists of a fresh air intake for the crankcase, hoses, and PCV valve. The PCV valve provides flow control for the introduction of crankcase vapors into the intake manifold and acts as a check valve in case of an engine backfire. It contains a spring loaded check valve that is operated by manifold vacuum. When the engine is at idle, vacuum overcomes the spring tension of the check valve and causes it to move inward and restrict vapor flow into the intake manifold. When the vehicle is cruising and vacuum is lower, the spring tension moves the check valve away from the seat, allowing higher flow of crankcase vapors into the intake manifold. Control of crankcase vapor flow is used to prevent excessive amounts of vapors into the intake manifold at idle speed. This could cause rough idle or incorrect idle speeds.
The PCV system operates by using intake manifold vacuum through the PCV valve to create negative pressure in the crankcase. Filtered air is drawn into the crankcase through a hose connected to the air cleaner or air intake duct. The flow of air is circulated through the crankcase and into the intake manifold, via the PCV valve. It then carries the crankcase vapors to the combustion chamber to be burned. The fresh air intake of the PCV system also serves to function as a pressure relief point for crankcase vapors, in the event of an internal engine malfunction or PCV valve blockage. If this occurs, excess vapors are vented through the air intake into the air cleaner. One symptom of crankcase vapors flowing into the fresh air intake is oil residue in the air filter housing, as any auto repair shop will tell you.
(Mark gave up on sports when the Browns left Cleveland and now spends his Sundays working under a shade tree in the back yard tuning his son’s soap box derby car.)