Nissan 240SX/Altima 1993-1998 Repair Guide

Crankcase Ventilation System

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OPERATION



The crankcase ventilation system returns blow-by gas to the intake manifold. A Positive Crankcase Ventilation (PCV) valve is used to meter the gas and is located in the breather separator under the intake manifold.

During partial throttle operation of the engine, manifold vacuum is high, allowing the blow-by gas to be sucked through the PCV valve into the intake manifold. Normally, the capacity of the valve is sufficient to handle any blow-by and a small amount of ventilating air. The ventilating air is drawn from the air duct into the crankcase. The ventilating air passes through the hose connecting the air inlet tube to the cylinder head cover.

Under full throttle condition, the manifold vacuum is insufficient to draw the blow-by flow through the valve. The flow reverses, coming from the crankcase, going through the ventilating hose and into the air duct.

COMPONENT TESTING



See Figures 1 and 2



Click image to see an enlarged view

Fig. Fig. 1: PCV system and airflow schematic



Click image to see an enlarged view

Fig. Fig. 2: Check the PCV valve for vacuum at idle

  1. Remove the PCV valve and shake it gently.
    1. If the valve rattles when shaken, connect the valve to the vacuum hose and proceed to Step 2.
    2.  
    3. If the valve does not rattle, it is sticking and must be replaced.
    4.  

  2.  
  3. Start the engine and allow it to reach normal operating temperature.
  4.  
  5. Check the PCV valve for vacuum by placing your finger over the end of the valve.
    1. If vacuum exists, proceed to Step 5.
    2.  
    3. If vacuum does not exist, check for loose hose connections, vacuum leaks or blockage. Correct as necessary.
    4.  

  6.  
  7. Reinstall the PCV valve.
  8.  
  9. With the engine running, disconnect the fresh air intake hose from the air inlet tube (which connects the air cleaner housing to the throttle body).
  10.  
  11. Place a stiff piece of paper over the hose end and wait 1 minute.
    1. If vacuum holds the paper in place, the system is OK; reconnect the hose.
    2.  
    3. If the paper is not held in place, check for loose hose connections, vacuum leaks or blockage. Correct as necessary.
    4.  

  12.  

REMOVAL & INSTALLATION



PCV Valve

See Figure 3



Click image to see an enlarged view

Fig. Fig. 3: The PCV valve, located under the intake manifold on the rear side of the engine, screws into an oil separator

The PCV valve is located under the intake manifold in the oil separator.

  1. Disconnect the vacuum hose from the PCV valve.
  2.  
  3. Unscrew the PCV valve from the breather separator under the intake manifold.
  4.  

To install:
  1. Screw the PCV valve into the breather separator under the intake manifold.
  2.  
  3. Connect the vacuum hose to the PCV valve.
  4.  

 
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