Toyota Previa 1991-1997 Repair Information

Positive Crankcase Ventilation (PCV) System

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OPERATION



See Figures 1 and 2

Exhaust blow-by gasses are routed from the crankcase to the intake manifold, where they are combined with the fuel/air mixture and burned during combustion. This reduces the amount of hydrocarbons emitted by the exhaust.

A PCV valve is used in the line to prevent the gases in the crankcase from being ignited in case of a backfire. The amount of blow-by gasses entering the mixture is also regulated by the PCV valve, which is spring loaded and has a variable orifice.

The important components of the PCV system are the following:



PCV valve
 
Valve cover
 
Air intake chamber
 
Ventilation case
 
Hoses, connections and gaskets
 



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Fig. Fig. 1: The PCV valve functions differently according to the engine operating conditions



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Fig. Fig. 2: PCV system flow

TESTING



See Figures 3, 4 and 5

Inspect the PCV system hoses and connections at each tune-up and replace any deteriorated hoses. Check the PCV valve at every tune-up and replace it at 30,000 mile (48,000 km) intervals.

The PCV valve is easily checked with the engine running at normal idle speed (warmed up).

  1. Remove the PCV valve from the valve cover or intake manifold, but leave it connected to its hose.
  2.  
  3. Start the engine.
  4.  



Click image to see an enlarged view

Fig. Fig. 3: Blow air from the cylinder head side of the PCV valve, it should flow easily

  1. Place your thumb over the end of the valve to check for vacuum. If there is no vacuum, check for plugged hoses or ports. If these are open, the valve is faulty.
  2.  
  3. With the engine OFF , remove the valve completely. Shake it end-to-end, listening for the rattle of the needle inside the valve. If no rattle is heard, the needle is jammed (probably due to oil sludge) and the valve should be replaced.
  4.  



Click image to see an enlarged view

Fig. Fig. 4: Blow air from the air manifold side, check that the air passes with difficulty


CAUTION
Don't blow directly into the valve; petroleum deposits within the valve can be harmful.

An engine without crankcase ventilation is quickly damaged. It is important to check the PCV at regular intervals. When replacing a PCV valve you must use the correct one for the engine. Many valves look alike on the outside, but have different mechanical values. Putting the incorrect valve on a vehicle can cause a great deal of driveability problems.



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Fig. Fig. 5: Leaks from any of these areas can cause the PCV system to malfunction

REMOVAL & INSTALLATION



  1. Remove the RH seat and No. 2 cylinder head cover.
    1. Remove the 3 screws and scuff plate.
    2.  
    3. Remove the bolt and disconnect the RH seat belt from the front floor panel.
    4.  
    5. Remove the 4 bolts and the RH front seat.
    6.  
    7. Extract the 2 bolts and the RH front seat leg.
    8.  
    9. Remove the 2 bolts and the jack holder.
    10.  
    11. Unbolt the RH engine service cover.
    12.  

  2.  
  3. Locate and remove the PCV valve.
  4.  
  5. Inspect the PCV valve operation.
  6.  
  7. Check the valve for proper operation. While the valve is removed, the hoses should be checked for splits, kinks and blockages. Check the vacuum port (that the hoses connect to) for any clogging.
  8.  
  9. Inspect the rubber grommet the PCV valve fits into. If it is in any way deteriorated or oil soaked, replace it.
  10.  

To install:
  1. Reinstall the PCV valve.
  2.  
  3. Attach the RH engine hole cover.
  4.  
  5. Install and secure the No. 2 cylinder head cover and RH seat.
    1. Tighten the bolts to the following specifications:

      Service hole cover bolts-10 ft. lbs. (14 Nm)
       
      Jack holder-10 ft. lbs. (14 Nm)
       
      RH seat leg-29 ft. lbs. (39 Nm)
       
      RH seat-29 ft. lbs. (39 Nm)
       
      RH seat belt-to-floor pan-31 ft. lbs. (42 Nm)
       

    2.  

  6.  

 
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