Positive Crankcase Ventilation: Description and Operation

The Positive Crankcase Ventilation System is provided to prevent the blow-by gas from being emitted from the crankcase into the atmosphere. The term "blow-by gas" stands for the compressed gas that blows through the cylinder-to-piston clearance, which contains a large amount of the gases CO and HC. During partial throttle opening, the manifold vacuum draws fresh air through the tube connected to the air cleaner, into the rocker cover and crankcase, thereby drawing the blow-by gas through the PCV valve into the intake air stream. During low vacuum conditions (full throttle or heavy engine load) the manifold vacuum is insufficient to draw the gases through the valve, so the gases flow in the reverse direction, being drawn into the air cleaner and then into the intake air stream. On engines with excessive blow-by some of the gases will flow into the air cleaner under all conditions.