Nissan Z - ZX 1970-1988 Repair Guide

Crankcase Ventilation System


See Figures 1 and 2

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Fig. Fig. 1: Typical crankcase emission control system-non-turbo engines

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Fig. Fig. 2: Typical crankcase emission control system-turbo engines


A closed, positive crankcase ventilation system is employed on all vehicles. This system cycles incompletely burned fuel, which works its way past the piston rings, back into the intake manifold for reburning with an fuel/air mixture. The oil filler cap is sealed and air is drawn from the top of the crankcase into the intake manifold through a valve with a variable orifice.

The valve (commonly known as the PCV valve) employs spring pressure and a sliding plunger to regulate the flow of air into the manifold according to the amount of manifold vacuum. When the throttle plate(s) are open fairly wide, this valve opens to maximize the flow. However, at idle speed, when manifold vacuum is at maximum, the PCV valve throttles the flow in order not to unnecessarily affect the small volume of mixture passing into the engine.

A ventilating line connects the valve cover with the air cleaner (carburetor) or the intake manifold (throttle body). During most driving conditions, manifold vacuum is high and the vapor from the crankcase, plus a small amount of excess air, is drawn into the manifold through the PCV. However, at full throttle, the increase in the volume of blow-by and the decrease in manifold vacuum make the flow through the PCV inadequate. Under these conditions, excess vapors are drawn into the air cleaner and pass through the carburetor and into the engine.


After every 12,000 miles (19,300 km) or every year, perform the following services:

  1. Check the condition of the hoses and the connectors to ensure that there is no leakage; replace the parts, if necessary.
  3. Disconnect the hoses and blow them clean with compressed air. Where extreme clogging is encountered, replace the hose.
  5. Check the PCV valve as follows:
    1. Start the engine and allow it to idle.
    3. Disconnect the ventilating hose from the PCV valve, allowing fresh air to be drawn into the manifold through the valve.


The flow of air should produce an audible "hiss'' and it should be possible to feel a strong vacuum when placing a finger over the valve inlet.

    1. If the valve is clogged, replace it as it is not serviceable. Replace the valve every two years.