Isuzu Cars and Trucks 1981-1991

Positive Crankcase Ventilation (PCV)


gasoline engines

The carbureted gasoline 4 cyl. engines do not use a PCV valve. In place of the valve is a regulating orifice in the intake manifold. Blow-by gases are drawn through the regulating orifice into the intake manifold for reburning. During wide open throttle, the engine vacuum is not sufficient to draw enough vapor through the manifold, allowing part to be drawn into the air cleaner via the rear end of the rocker arm cover.

The fuel injected gasoline 4 cyl. and V6 engines use a PCV valve. The PCV valve meters the flow at a rate depending upon the manifold vacuum. If the manifold vacuum is high, the PCV restricts the flow to the intake manifold. If abnormal, operating conditions occur, excessive amounts of internal exhaust gases back flow through the crankcase vent tube into the air filter to be burned by normal combustion.

The crankcase ventilation system (PCV) must be operating correctly to provide complete scavenging of the crankcase vapors. Fresh air is supplied to the crankcase from the air filter, mixed with the internal exhaust gases, passed through the PCV valve or orifice and into the intake manifold.

If the engine is idling roughly, a quick check of the PCV valve can be made. While the engine is idling, pull the PCV valve from the valve cover, place your thumb over the end of the PCV valve and check for vacuum. If no vacuum exists, check for a plugged PCV valve, manifold port, hoses or deteriorated hoses. Turn the engine "OFF", remove the PCV valve and shake it. Listen for the rattle of the check needle inside the valve. If it does not rattle, replace the valve.

The PCV system should be checked at every oil change and serviced every 30,000 miles.

Never operate an engine without a PCV valve or a ventilation system, for it can become damaged.

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Fig. PCV system carbureted 4-cylinder engines do not use a valve. They use a regulating orifice located in the intake manifold

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Fig. PCV system 4-cylinder gas EFT 4ZE1 engine

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Fig. PCV system 6-cylinder engine


Check the diaphragm valve for damage and adhesion to the seating surface. Check the oil separator element for wear if any abnormal conditions are found, replace the PCV valve assembly.

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Fig. PCV system 4-cylinder diesel engine