POSITIVE CRANKCASE VENTILATION (PCV)

When troubleshooting cars which fail an emissions inspection, one of the most common culprits is the PCV valve. When an engine is running, a certain amount of combustion gas escapes past the rings into the crankcase; this is normal. Combustion gases consists mostly of carbon dioxide and water vapor, but do contain small amounts of unburned fuel and other contaminants. Water and fuel vapors in the crankcase can react to rapidly break down and shorten the life of engine oil. In addition, these vapors contribute to the formation of sludge in the crankcase. Incidentally, this sludge should be cleaned out periodically. Any auto repair garage can perform this for you. In the past, a tube was installed that vented crankcase vapors to the atmosphere while the vehicle was moving. This tube was called the road draft tube. Later it was discovered that a large portion of the emissions of a vehicle were produced by crankcase vapors vented into the atmosphere. This resulted in the development of the PCV system and the disappearance of road draft tubes on engines.

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.