Chrysler Full-Size Trucks 1989-1996 Repair Guide

Crankcase Ventilation System



Except 8.0L Engine

See Figures 1, 2, 3, 4, 5 and 6

All 6.9L, 7.9L, 9.6 and 9.9L engines are equipped with a Closed Crankcase Ventilation (CCV) system and a Positive Crankcase Ventilation (PCV) valve. The 8.0L engine is not equipped with a PCV valve, but its CCV system otherwise operates on the same principle and performs the same function.

On all but the 8.0L engine, the crankcase emission control equipment consists of PCV valve mounted on the cylinder head cover, with a hose extending from the valve to the base of the throttle body. A closed engine crankcase inlet air filter with a hose connecting it to the throttle body air cleaner housing provides the source of air for the system.

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Fig. Fig. 1: Fresh air enters the crankcase via the crankcase inlet air filter

When the engine is running, a small portion of the gases which are formed in the combustion chamber leak past the piston rings and enter the crankcase. Since these gases are under pressure they tend to escape from the crankcase and enter into the atmosphere. If these gases are allowed to remain in the crankcase for any length of time, they would contaminate the engine oil and cause sludge to build up. If the gases are allowed to escape into the atmosphere, they would pollute the air, as they contain unburned hydrocarbons. The crankcase emission control equipment recycles these gases back into the engine combustion chamber, where they are burned.

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Fig. Fig. 2: A common Positive Crankcase Ventilation (PCV) system-V8 engines

The PCV system operates by manifold vacuum. Air is drawn from the throttle body air cleaner hose to the crankcase inlet air filter, passing through the filter into the crankcase. The air circulates inside the engine and is drawn out through the PCV valve. The gases pass through the PCV hose and into the throttle body where they become part of the air/fuel mixture.

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Fig. Fig. 3: Positive closed Crankcase Ventilation (PCV) system-6.9L engine

The most critical component of the system (except for 8.0L engine) is the PCV valve. This vacuum-controlled valve regulates the amount of gases which are recycled into the combustion chamber. At low engine speeds the valve is partially closed, limiting the flow of gases into the intake manifold. As engine speed increases, the valve opens to admit greater quantities of the gases into the intake manifold. If the valve should become blocked or plugged, the gases will be prevented from escaping the crankcase by the normal route. Since these gases are under pressure, they will find their own way out of the crankcase. This alternate route is usually a weak oil seal or gasket in the engine. As the gas escapes by the gasket, it also creates an oil leak. Besides causing oil leaks, a clogged PCV valve also allows these gases to remain in the crankcase for an extended period of time, promoting the formation of sludge in the engine.

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Fig. Fig. 4: Positive Crankcase Ventilation (PCV) system-V6 engine

The fixed orifice tube is gray in color and is mounted on top of the right cylinder head cover. Do not confuse it with a similar black fitting on the left cylinder head (valve) cover.

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Fig. Fig. 5: A common crankcase inlet air filter

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Fig. Fig. 6: The PCV valve fits into the sealing grommet as shown

8.0L Engine

In lieu of a PCV valve, the 8.0L engine makes use of a (gray) molded vacuum tube that connects manifold vacuum to the top of the right cylinder head (valve) cover. The vacuum tube connects to a fixed orifice fitting of a calibrated size of 0.90 in. (6.6mm). This fitting meters the amount of crankcase vapors drawn out of the engine.

On the left cylinder head cover of the 8.0L engine is a similar black fitting that does not contain a fixed orifice. Do not interchange the gray and the black fittings.

The 8.0L engine CCV system operates as follows: When the engine is running, fresh air enters the engine and mixes with the crankcase vapors. This mixture of vapors and outside air is then drawn by manifold vacuum through the fixed orifice tube into the intake manifold and into the combustion chamber where it is burned.


With the engine running, pull the PCV valve and hose from the valve rocker cover rubber grommet.

If the valve is working properly, a hissing noise should be heard as air passes through the valve and a strong vacuum should be felt when you place a finger over the valve inlet. While you have your finger over the PCV valve inlet, check for vacuum leaks in the hose and at the connections.

When the PCV valve is removed from the engine, a metallic clicking noise should be heard when it is shaken. This indicates that the metal check ball inside the valve is still free and is not gummed up.


PCV Valve
  1. Pull the PCV valve and hose from the rubber grommet in the rocker cover.
  3. Remove the PCV valve from the hose. Inspect the inside of the PCV valve. If it is dirty, disconnect it from the intake manifold and clean it in a suitable, safe solvent.

To install:
  1. If the PCV valve hose was removed, connect it to the intake manifold.
  3. Connect the PCV valve to its hose.
  5. Install the PCV valve into the rubber grommet in the valve rocker cover.