Jeep Wrangler/YJ 1987-1995 Repair Guide

PCV Valve



See Figures 1 and 2

The PCV valve, used on 1987-90 models, controls the rate at which crankcase vapors are returned to the intake manifold. The action of the valve plunger is controlled by intake manifold vacuum and the spring. During deceleration and idle, when manifold vacuum is high, it overcomes the tension of the valve spring and the plunger bottoms in the manifold end of the valve housing. Because of the valve construction, it reduces, but does not stop, the passage of vapors to the intake manifold. When the engine is lightly accelerated or operated at constant speed, spring tension matches intake manifold vacuum pull and the plunger takes a mid-position in the valve body, allowing more vapors to flow into the manifold.

The PCV valve, which is the heart of the positive crankcase ventilation system, should be free of dirt and residue and in working order. As long as the valve is kept clean and is not showing signs of damage or gumming up, it should work properly. When the valve cannot be cleaned sufficiently or becomes sticky and will not operate freely, it should be replaced.

The PCV system used on vehicles with a 4.2L engine, employs a PCV shut-off solenoid that is controlled by the engine computer. Air flow through the PCV valve is controlled by the PCV shut-off solenoid and manifold vacuum. The solenoid is located in the hose between the PCV valve where the hose connects to the intake manifold vacuum. When the PCV valve shut-off solenoid is not energized, crankcase ventilation is not restricted.

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Fig. Fig. 1: Positive crankcase ventilation system-2.5L engine

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Fig. Fig. 2: Positive crankcase ventilation system-4.2L engine


An inoperative PCV system will cause rough idling, sludge and oil dilution. In the event erratic idle, never attempt to compensate by disconnecting the PCV system. Disconnecting the PCV system will adversely affect engine ventilation. It could also shorten engine life through the buildup of sludge.

The PCV valve is in the the rocker arm cover. To inspect the PCV valve, proceed as follows:

  1. With the engine idling, remove the PCV valve from the rocker cover. If the valve is not plugged, a hissing sound will be heard. A strong vacuum should be felt when you place your finger over the valve.
  3. Reinstall the PCV valve and allow about a minute for pressure to drop.
  5. Remove the crankcase intake air cleaner. Cover the opening in the rocker cover with a piece of stiff paper. The paper should be sucked against the opening with noticeable force.
  7. With the engine stopped, remove the PCV valve and shake it. A rattle or clicking should be heard to indicate that the valve is free.
  9. If the system passes the tests in Steps 1-4, no further service is required, unless replacement is specified in the Maintenance Intervals Chart. If the system does not pass these tests, the valve should be replaced with a new one.

Do not attempt to clean a PCV valve.

  1. With a new PCV valve installed, if the paper is not sucked against the crankcase air intake opening (see Step 2), it will be necessary to clean the PCV valve hose and the passage in the lower part of the carburetor.
  3. Clean the line with Combustion Chamber Conditioner or similar solvent. Do not leave the hoses in solvent for more than 1 / 2 hour. Allow the line to air dry.
  5. Remove the carburetor or throttle body and HAND turn a 1 / 4 in. drill through the passages to dislodge solid particles, then blow clean.

It is not necessary to disassemble the carburetor for this operation. If necessary, use a smaller drill, so that no metal is removed.

  1. After checking and/or servicing the Crankcase Ventilation System, replace any components that do not allow passage of air to the intake manifold.