Positive Crankcase Ventilation: Description and Operation

Positive Crankcase Ventilation:

SYSTEM DESCRIPTION
The Positive Crankcase Ventilation (PCV) system consists of two vacuum/pressure operated valves and associated hoses and plumbing. The system prevents blow-by gases from escaping into the atmosphere by routing them through a vacuum controlled ventilation valve, into the intake manifold. The blow-by gases combine with the air/fuel mixture and are burned in the combustion chamber with the normal air/fuel charge.

The PCV valve, consists of a needle valve, spring and housing. When the engine is off, the spring holds the needle valve closed to stop vapors from entering the intake manifold. When the engine is running, manifold vacuum lifts the valve off its seat and allows crankcase vapors to enter the intake manifold. To maintain idle quality, the PCV valve restricts the flow when the intake manifold vacuum is high. In case of a backfire in the intake manifold, the valve closes. This stops the backflow of gases and prevents the ignition of fuel vapor in the crankcase.

OPERATION
When the engine is running under light load, there is little blow-by into the crankcase. Fresh air is drawn into the crankcase through vent hoses connected to the throttle body extension. This air mixes with the blow-by gases and oil vapors in the crankcase to help purge the crankcase of harmful fumes. During heavy load, such as hard acceleration or climbing steep hills, more blow-by gases are produced than the PCV valve can handle. In this case, the excess blow-by gases are vented through the hoses to the throttle body extension, so they can be drawn into the engine and burned.