Positive Crankcase Ventilation: Description and Operation

Crankcase Ventilation Valve Cross Section:

PCV Valve Exploded View

PCV Flow:

PCV Flow


The PCV system prevents blow-by gases from escaping by routing them through a vacuum controlled ventilation valve and a hose into the intake manifold. The blow-by gases mix with the air/fuel mixture and are burned in the combustion chambers. When the engine is running, fresh air is drawn into the crankcase through a tube or hose connected to the air cleaner housing.

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 unseats the valve allowing crankcase vapors to enter the intake manifold. In case of a backfire in the intake manifold, the valve closes, stopping the backflow and preventing ignition of fumes in the crankcase. During certain engine conditions, more blow-by gases are created than the ventilator valve can handle. The excess is returned through the air intake tube to the air cleaner and carburetor where it is burned in the engine.

PCV System:

The PCV system used on this engine routes the blow by gases through an oil air separator and into the intake manifold. Unlike conventional PCV systems, it does not provide a fresh air inlet into the crankcase. Blow by gases flow through a hose from the timing chain housing into the separator, where oil is separated and drained back into the crankcase through an oil fill tube. A hose connected to manifold vacuum draws the gases from the separator at high vacuum conditions such as at idle and steady cruise conditions. Another hose, connected to the air intake duct of the air cleaner, draws gases from the separator under high air flow or low vacuum conditions.