Chrysler Full-Size Trucks 1967-1988 Repair Guide

Crankcase Ventilation System



See Figures 1 through 8

The Positive Crankcase Ventilation (PCV) system is used in two forms, the standard and closed types. The standard type is used on all applications except 1967 California models, while the closed type is used exclusively on 1967 California models.

Blowby gasses or crankcase vapors must be removed from the crankcase to prevent oil dilution and to prevent the formation of sludge. Traditionally, this was accomplished with a road draft tube. Air entered the rocker arm cover through an open oil filler cap and flowed down past the pushrods, mixing with the blow-by gases in the crankcase. It was finally routed into the road draft tube where a partial vacuum was created, drawing the mixture into the road draft tube and out into the atmosphere.

The open PCV system replaced the road draft tube and engine manifold vacuum was used instead of the action of a moving vehicle to create a low pressure area. Air flowed into an open oil filler cap, which is characteristic of an open PCV system, and mixed with the crankcase fumes. These vapors were then drawn into the intake manifold and burned in the combustion chamber. Under heavy acceleration, however, manifold vacuum would decrease and crankcase pressure would build up. When this happened, a portion of the crankcase vapors were forced back out of the oil filter cap, creating a system which was only about 75% efficient.

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Fig. Fig. 1: Common six cylinder PCV system

The closed PCV system operates in a similar manner as the open system, except that a sealed oil filler cap and dipstick are used in place of a vented oil filler cap. In addition, an air intake hose is installed between the carburetor air filter and crankcase opening in the valve cover. A separate PCV air filter is used when the air intake hose is connected to the "dirty'' side of the carburetor air cleaner. This filter is located where the intake air line connects to the valve cover.

Under normal engine operation, the closed PCV system operates the way an open system does, except that air enters through the intake hose via the air filter. Under heavy acceleration, any excess vapors back up through the air intake hose. They are forced to mix with incoming air from the carburetor and are burned in the combustion chamber. Back-up fumes cannot escape into the atmosphere, thereby creating a closed system.

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Fig. Fig. 2: Common V8 engine PCV system

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Fig. Fig. 3: Fully closed crankcase ventilation system on 6-225 and 8-318 engines-1967 vehicles

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Fig. Fig. 4: Partially closed crankcase ventilation system on 6-225, 8-318 and 8-383 engines-1967 vehicles

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Fig. Fig. 5: Remove the crankcase intake air filter hose ...

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Fig. Fig. 6: ... then pull the filter from the retaining grommet

The PCV valve is used to control 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.

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Fig. Fig. 7: Fully closed crankcase ventilation system on 8-383 engine-1967 vehicles

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Fig. Fig. 8: Fully closed crankcase ventilation system on 8-413 engine-1967 vehicles

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


See Figure 9

  1. With the engine idling, remove the PCV valve from the rocker arm 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.

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Fig. Fig. 9: Place your finger over the valve to check for vacuum

  1. Reinstall the PCV valve and allow about a minute for pressure to drop.
  3. Remove the crankcase intake air cleaner. Cover the opening in the rocker arm cover with a piece of stiff paper. The paper should be sucked against the opening with noticeable force.
  5. With the engine stopped, remove the PCV valve and shake it. A rattle or clicking should be heard to indicate that the valve is unobstructed.
  7. Install the new PCV valve into the grommet.
  9. With a new PCV valve installed, if the paper is not sucked against the crankcase air intake opening, it will be necessary to clean the PCV valve hose and the passage in the lower part of the carburetor. If the hose is cracked, oil soaked, or in otherwise poor condition, replace the hose with a new one.

If necessary, clean the hose with Combustion Chamber Conditioner or similar solvent, then allow it to dry. Do not leave the hose in solvent for more than1/2hour.

  1. If the carburetor passage is blocked, remove the carburetor, as described in 1 / 4 in. drill through the passage to dislodge solid particles, then blow clean. Fuel System . Hand turn a

It is necessary to disassemble the carburetor for this operation. If necessary, use a smaller drill, so that no metal is removed.

  1. Remove the crankcase intake air cleaner and wash it thoroughly in kerosene or similar solvent. Lubricate the filter by inverting it and filling with SAE 30 engine oil. Position the filter to allow excess oil to drain thoroughly through the vent nipple.


See Figure 10

  1. With the engine OFF , remove the PCV valve by disconnecting the vacuum hose attached to it.

Do not attempt to clean a clogged PCV valve.

  1. Inspect the PCV valve's rubber grommet and hoses for any excessive wear or cracks, and replace them if necessary. Be careful when replacing the grommet that you do not drop any pieces of rubber into the rocker arm cover. You can use needlenose pliers to remove the grommet.

To install:
  1. Apply lubricant on the outside of the grommet (if a new one is to be installed) to ease installation.
  3. Install the new PCV valve into the grommet, then connect the vacuum hose.

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Fig. Fig. 10: Common PCV location

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