GM S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual

Positive Crankcase Ventilation

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OPERATION



See Figure 1

Before the years covered by this manual, Federal law mandated that oil fill caps on engines be sealed to prevent the escape of crankcase vapors into the atmosphere. The result was a closed crankcase ventilation system which is designed to control crankcase pressure, while preventing it from escaping into the atmosphere.

To supply this closed system with fresh air, a breather hose runs from the carburetor air cleaner to an inlet hole in the valve cover. The carburetor or throttle body end of the hose usually fits into a cup-shaped flame arrestor and breather filter in the air cleaner cover. In the event of a carburetor backfire, this arrestor prevents the spread of fire to the valve cover where it could create an explosion.



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

Included in the closed system is a Positive Crankcase Ventilation (PCV) valve that is usually mounted into an outlet hole in the top of the valve cover. A hose or hoses connects this valve to a vacuum outlet at the throttle body or intake manifold. Contained within the PCV housing is a unidirectional valve (pointed at one end, flat at the other) positioned within a coiled spring. The PCV valve meters the air flow rate, which varies under engine operation, depending on manifold vacuum. During idle or low speed operation, when manifold vacuum is highest, the valve spring tension is overcome by the high vacuum pull and, as a result, the valve is pulled up to very nearly seal off the manifold end of the valve housing. This restricts the flow of crankcase vapors to the intake manifold at a time when crankcase pressures are lowest and least disruptive to engine performance. The restricted flow also prevents the crankcase vapors from altering the idle mixture and thereby disrupting idle quality. At times of acceleration or constant speed, intake manifold vacuum is reduced to a point where it can no longer pull against the valve spring and so, spring force pulls the valve away from the housing outlet allowing crankcase vapors to escape through the hose to the intake manifold. Once inside the manifold, the gases enter the combustion chambers to be reburned. At times of engine backfire (at the carburetor), manifold vacuum ceases permitting the spring to pull the valve against the inlet (crankcase end) end of the valve housing. This seals off the inlet, thereby stopping the entrance of crankcase gases into the valve and preventing the possibility of a backfire spreading through the hose and valve to ignite these gases.

On the 1.9L engine, the blow by gases are forced back into the intake manifold through a closed system which consists of a baffle plate and an orifice mounted in the intake manifold.

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.

Inspect the PCV system hose(s) 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 intervals.

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.  



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Fig. Fig. 2: To test for vacuum, remove the PCV valve from the grommet, with the hose attached

  1. 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.
  2.  

REMOVAL & INSTALLATION



See Figures 3 through 7

PCV Valve

The large end of the valve is inserted into a rubber grommet in the valve cover. At the narrow end, it is inserted into a hose and usually clamped. To remove it, gently pull it out of the valve cover, then open the clamp with a pair of pliers. Hold the clamp open while sliding it an inch or two down the hose (away from the valve), and then remove the valve.

If the end of the hose is hard or cracked where it holds the valve, it may be feasible to cut the end off if there is plenty of extra hose. Otherwise, replace the hose. Replace the grommet in the valve cover if it is cracked or hard. Replace the clamp if it is broken or weak. In replacing the valve, make sure it is fully inserted in the hose, that the clamp is moved over the ridge on the valve so that the valve will not slip out of the hose, and that the valve is fully inserted into the grommet in the valve cover.



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Fig. Fig. 3: PCV system schematic for the 1.9L engine



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Fig. Fig. 4: PCV system-2.5L engine



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Fig. Fig. 5: PCV valve location and mounting-2.8L engine



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Fig. Fig. 6: PCV system-4.3L MFI-Turbo engine



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Fig. Fig. 7: PCV system-4.3L (VIN Z, except Turbo), VIN W similar

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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