Positive Crankcase Ventilation: Description and Operation

Fig. 1 Positive crankcase ventilation system.:

Fig. 2 Positive crankcase ventilation system.:

This system, Fig. 1 and 2, leads the blow-by gasses collected in the crankcase to the intake manifold, where they are then drawn into the combustion chamber and burned with the normal air/fuel mixture.

On carbureted models it consists of a hose connecting the air cleaner to the crankcase, and a hose connecting the crankcase to the intake manifold with a ventilation valve connected to it.

Air enters the system from the air cleaner to the cylinder head cover. Fresh air mixes with crankcase fumes and enters the ventilation tube and into ventilation valve. The ventilation valve regulates the amount of air flow to meet various operating conditions. The air is then drawn into the intake manifold, through a connecting hose, where it enters the combustion system and is burned.

The ventilation valve is operated by pressure differential between the crankcase and intake manifold. With no pressure differential (the engine not operating) or when the pressure of the intake manifold is greater than that of the crankcase, the valve is closed. During engine idle, high intake manifold vacuum overcomes valve spring tension, and is pulled toward the intake manifold side by vacuum. This causes air to pass through the restricted passage between the valve and the housing. During normal engine operation, the valve remains in a position where spring pressure and intake manifold vacuum balance. The amount of air flow then depends on the position of the valve.

On fuel injected models except 2S-E, 3Y-EC, 4Y-EC and 22R-E, the engine is sealed, and the PCV system consists of two metering orifices, a baffle plate in the top of the valve cover, and hoses, routing the blow-by gasses into the intake stream. The hose connected to the baffle in the valve cover, is connected to the two orifices. One orifice is ported to the intake stream before the throttle plate, and the other orifice is installed after the throttle. This provides a constant vacuum to evacuate the crankcase, and blow-by gasses are drawn into the engine to be burned.

The 2S-E, 3Y-EC, 4Y-EC and 22R-E engines use a PCV valve and operate as do other PCV valve-equipped vehicles.