Positive Crankcase Ventilation: Description and Operation

Carbureted Models

Fig. 001 - Crankcase Control System:

This system returns blow-by gasses both to the intake manifold and to the carburetor air cleaner, Fig. 1. A valve controlled by the carburetor throttle shaft conducts crankcase blow-by gasses into the intake manifold. Blow-by gasses are ducted from the crankcase through a liquid/vapor separator and a tube which is connected to the carburetor air cleaner. The vacuum line from the control valve is connected to the tube at the air cleaner.

When the throttle is closed, blow-by gasses are drawn into the intake manifold through a calibrated orifice in the control valve, Fig. 1. As the throttle opens, the size of the orifice increases, increasing the amount of vacuum applied to the system. At full throttle, manifold vacuum is insufficient to draw all of the blow- by gasses into the manifold. The excess is drawn into the clean side of the air cleaner.


Fuel injected Models

Fig. 2 - Crankcase Emission Control System:

This system returns blow-by both to the intake manifold and the throttle chamber. Blow-by gasses are ducted from the crankcase through a liquid/vapor separator and a tube which is connected to the throttle chamber, Fig. 2. On 1500 engines a vacuum line is routed from the intake manifold to the tube to bypass the throttle plate during idling.

When the throttle plate is closed, blow-by gasses are drawn into the intake manifold through the vacuum line. As the throttle plate opens, suction of the intake stream draws blow-by gasses into the engine to be reburned.