Gasoline Models
The crankcase ventilation system is designed to return blow-by gases to engine through the intake manifold and air cleaner. Gasoline models use a PCV valve to control blow-by flow.
During part throttle engine operation, intake vacuum pulls blow-by gases through PCV valve. Capacity of valve is sufficient to handle blow-by gas plus some ventilating air. Ventilating air is drawn from carburetor air cleaner, or from air duct on fuel injected models, and into crankcase through connecting tube.
During wide open throttle, manifold vacuum is not sufficient to draw blow-by gases through valve. These gases then go through tube to air cleaner in a reverse direction. Engines with excessive blow-by will pass crankcase gases through tube to carburetor during all operating conditions.