GM S-Series Pick-ups and SUV's 1994-1999 Repair Guide

Positive Crankcase Ventilation

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OPERATION



See Figure 1

The Positive Crankcase Ventilation (PCV) system is used to evacuate the crankcase vapors. Outside vehicle air is routed through the air cleaner to the crankcase where it mixes with the blow-by gases and is passed through the PCV valve. It is then routed into the intake manifold. The PCV valve meters the air flow rate which varies under engine operation depending on manifold vacuum. In order to maintain idle quality, the PCV valve limits the air flow when intake manifold vacuum is high. If abnormal operating conditions occur, the system will allow excessive blow-by gases to back flow through the crankcase vent tube into the air cleaner. These blow-by gases will then be burned by normal combustion. A plugged PCV valve or hose may cause rough idle, stalling or slow idle speed, oil leaks, oil in the air cleaner or sludge in the engine. A leaking PCV valve or hose could cause rough idle, stalling or high idle speed. Other than checking and replacing the PCV valve and associated hoses, there is no service required. Engine operating conditions that would direct suspicion to the PCV system are rough idle, oil present in the air cleaner, oil leaks and excessive oil sludging or dilution. If any of the above conditions exist, remove the PCV valve and shake it. A clicking sound indicates that the valve is free. If no clicking sound is heard, replace the valve. Inspect the PCV breather in the air cleaner. Replace the breather if it is so dirty that it will not allow gases to pass through. Check all the PCV hoses for condition and tight connections. Replace any hoses that have deteriorated.



Click image to see an enlarged view

Fig. Fig. 1: Positive Crankcase Ventilation (PCV) system schematic

TESTING



See Figure 2

  1. Remove the PCV valve from the rocker arm cover, but leave the vacuum hose attached.
  2.  
  3. Operate the engine at idle speed.
  4.  
  5. Place your thumb over the end of the valve to check for vacuum. If no vacuum exists, check the valve, the hoses or the manifold port for a plugged condition.
  6.  
  7. Remove the valve from the hose(s), then shake it and listen for a rattling of the check needle (inside the valve); the rattle means the valve is working. If no rattle is heard the valve is stuck, and should be replaced.
  8.  



Click image to see an enlarged view

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

REMOVAL & INSTALLATION



Refer to Routine Maintenance of this repair guide for PCV valve removal & installation.

PCV Breather

Most breathers are located inside the air cleaner assembly, though they may be mounted directly to a valve cover. Although a breather may in some cases be removed and cleaned, it is an inexpensive part and it is wise to replace it if dirty. Breathers which are mounted directly to the valve cover may be simply grasped and pulled from the cover grommet. For breathers which are mounted inside the air cleaner follow the procedure listed below.

  1. Loosen the wing nut or release the retainers, then remove the top of the air cleaner assembly.
  2.  
  3. Slide the rubber coupling that joins the tube coming from the valve cover to the breather off the breather nipple.
  4.  
  5. Slide the spring clamp off the breather nipple (if equipped) which is protruding from the air cleaner housing, then withdraw the breather from inside the air cleaner assembly.
  6.  
  7. Inspect the rubber grommet in the valve cover and the rubber coupling for brittleness or cracking. Replace parts as necessary.
  8.  

To install:
  1. Insert the new PCV breather through the hole in the air cleaner with the open portion of the breather upward. Make sure that the breather is fully seated in the air cleaner housing.
  2.  
  3. If equipped, install a new spring clamp onto the nipple. Make sure the clamp goes under the ridge on the breather nipple all the way around.
  4.  
  5. Reconnect the rubber coupling.
  6.  
  7. Install the air cleaner cover and secure.
  8.  

 
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