Nissan Pick-ups and Pathfinder 1970-1988

Positive Crankcase Ventilation (PCV) System



See Figures 1, 2 and 3

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

The crankcase emission control system on diesel engines is basically the same as that which is on the gasoline engine. Its major difference is the crankcase emission control valve. Although its function is the same as the gasoline engine's PCV valve, it's shape and location are different.

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 allowed to remain in the crankcase for any length of time, they would contaminate the engine oil and cause sludge to build up. If the gases are 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 recycled 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 chamber. 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 the gases into the intake manifold. If the valve should become blocked or plugged, the gases will be prevented from escaping out of 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 or gasket or both if necessary to ensure proper sealing.

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Fig. Fig. 1: Positive crankcase ventilation system-1981-86 Z20, Z22 and Z24 engines

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Fig. Fig. 2: Positive crankcase ventilation system-SD25 engines

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Fig. Fig. 3: Positive crankcase ventilation system-1986-88 Z24i engines


See Figures 4, 5, 6, 7 and 8

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

Gasoline Engine

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 may be made 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.

If the valve fails to function as outlined replace with a new one.

Do not attempt to clean or adjust the valve. Replace it with a new one.

Diesel Engine

Remove the air control valve and plug the center hole with your finger or a piece of tape. On 49 State and Canada trucks, blow air into the inlet pipe and check that it comes out the outlet pipe; additionally, when sucking on the inlet pipe there should be no air flow. On California models, suck on the outlet pipe and check that air flows freely from the inlet pipe; additionally, block the inlet pipe and then suck on the outlet pipe. You should be able to hear the diaphragm in the valve click open while you are sucking. If the valve fails to function as detailed, replace it with a new one.

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Fig. Fig. 4: Testing the PCV valve-4 cylinder gasoline engines

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Fig. Fig. 5: Clean out the PCV valve

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Fig. Fig. 6: Testing the PCV valve-VG30i engines

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Fig. Fig. 7: Testing the PCV valve-SD25 engines (49 state & Canada)

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