Crankcase Ventilation Systems: Operation

Air enters system through oil breather cap on 6 cyls. and through a breather on valve cover on V8 engines, and is circulated through engine. After circulating through engine air is returned to intake manifold through an exhaust tube which extends from crankcase ventilation outlet in valve cover to a regulator valve. This valve regulates amount of air to meet changing operating conditions. When engine is stopped the valve is closed and prevents engine fumes from entering crankcase. In case of a backfire the valve is forced closed. The regulator valve is controlled by intake manifold vacuum. During idle, intake manifold vacuum is high. High vacuum overcomes the valve and seats the valve. With valve in this position, all ventilating air passes through a calibrated orifice in valve, and there is minimum ventilation. As engine speed increases and manifold vacuum decreases, the valve moves off its seat and to a full open position which increases flow of ventilating air.