Nissan Pick-ups and Pathfinder 1989-1995

Positive Crankcase Ventilation (PCV) System



See Figures 1 and 2

The crankcase emission control equipment consists of a Positive Crankcase Ventilation (PCV) valve, a closed filler cap and hoses to connect this equipment.

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Fig. Fig. 1: PCV system-4 cylinder engine

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Fig. Fig. 2: PCV system-6 cylinder engine

When the engine is running, a small portion of the gases which are formed in the combustion chamber during combustion leak by the piston rings and enter the crankcase. Since these gases are under pressure they tend to escape from the crankcase and enter into the atmosphere. If these gases were sealed in the crankcase for any length of time, they would contaminate the engine oil and cause sludge to build up. If the gases were allowed to escape into the atmosphere, they would pollute the air, as they contain unburned hydrocarbons. The crankcase emission control equipment recycles these gases back into the engine combustion chamber where they are burned.

Crankcase gases are used in the following manner: while the engine is running, clean filtered air is drawn into the crankcase through the air filter and then through a hose leading to the rocker cover. As the air passes through the crankcase it picks up the combustion gases and carries them out of the crankcase, up through the PCV valve and into the intake manifold. After they enter the intake manifold they are drawn into the combustion chamber and burned.

The most critical component in the system is the PCV valve. This vacuum controlled valve regulates the amount of gases which are recycled into the combustion changer. At low engine speeds the valve is partially closed, limiting the flow of gases into the intake manifold. As engine speed increases, the valve opens to admit greater quantities of gases into the intake. If the valve should become blocked or plugged, the gases will be prevented from escaping from the crankcase by the normal route. Since these gases are under pressure, they will find their own way out of the crankcase. This alternate route is usually a weak oil seal or gasket in the engine. As the gas escapes by the gasket it also creates an oil leak. Besides causing oil leaks, a clogged PCV valve also allows these gases to remain in the crankcase for an extended period of time, promoting the formation of sludge in the engine.

The PCV system will not function properly unless the oil filler cap is tightly sealed. Check the gasket on the cap and be certain it is not leaking. Replace the cap, gasket or both as necessary to ensure proper sealing.


See Figures 3, 4, 5 and 6

Check the PCV valve, system hoses, retaining clamps and connections, to see that there are no leaks. Then replace or tighten, as necessary.

Valve Assembly

To check the valve, remove it and blow through both of its ends. When blowing from the side which goes toward the intake manifold, very little air should pass through it. When blowing from the crankcase (cylinder head cover) side, air should pass through freely.

An additional check without removing the valve could be with the engine running, remove the ventilator hose from the PCV valve. If the valve is working, a hissing noise will be heard as air passes through the valve and a strong vacuum should be felt immediately when the valve inlet is blocked with a finger. If the valve is suspected of being plugged, it should be replaced.

Replace the valve with a new one if the valve fails to function as outlined.

After removing the PCV valve shake it end to end, listening for the rattle of the needle inside the valve. If no rattle is heard, the needle and valve is jammed. Do not attempt to clean or adjust the valve, replace it.

System Hoses

Check all hoses and connections for leaks. Disconnect all hoses and clean them with compressed air. If the hose cannot be freed of its obstruction with the force of air, replace the hose.

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Fig. Fig. 3: Testing the PCV valve-Z24i engine

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Fig. Fig. 4: Testing the PCV valve-VG30i engine

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Fig. Fig. 5: A strong vacuum should be felt when testing the PCV valve attached to the system hose with engine running

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Fig. Fig. 6: Use compressed are to checking the PCV hoses for restrictions


No adjustments are either necessary or possible on the PCV system.


Valve Assembly

To remove the PCV valve, simply loosen the hose clamp and remove the valve from the manifold-to-crankcase hose and intake manifold. Most valves pull right out, although some may require unscrewing.

PCV Filter

Remove the air cleaner housing lid. Unclip the PCV filter from the side of the housing and then replace it with a new one. Install the housing lid.