When the engine is running, a small portion of the gases which are formed in the combustion chamber will leak by the piston rings, thus entering the crankcase. Since these gases are under pressure they tend to escape the crankcase and enter into the atmosphere. If the gases were allowed to remain in the crankcase for any length of time, they would contaminate the engine oil and cause sludge to build. If the gases were allowed to escape into the atmosphere, they would pollute the air, as they contain unburned hydrocarbons. The crankcase ventilation system recycles these gases back into the combustion chambers, where they are burned.
Crankcase gases are recycled in the following manner. While the engine is running, fresh air is drawn into the engine and mixes with crankcase vapors. Manifold vacuum draws the crankcase vapors up into the intake and they are burned during the engines normal combustion.
The Positive Crankcase Ventilation (PCV) system utilizes a vacuum controlled PCV valve located in the valve cover or the oil filler housing (depending on model). This valve regulates the amount of gases that 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.
The Closed Crankcase Ventilation (CCV) system operates the same way as a PCV system, however it does not utilize a vacuum controlled valve. A fitting of a calibrated size, referred to as a fixed orifice, meters the amount of crankcase vapors that the engine burns. This fitting can be found located in the valve covers of engines that employ this system. No maintenance is required.
The 3.9L, 4.7L, 5.2L and 5.9L gasoline engines are fitted with a Positive Crankcase Ventilation (PCV) system.
The 2.5L and 8.0L engines are fitted with a Crankcase Ventilation (CCV) system. It performs the same function as the PCV system described above, but does not use a vacuum-controlled valve.
Description & Operation
When the engine is running, a small portion of the gases which are formed in the combustion chamber leak by 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 were 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.
Crankcase gases are recycled in the following manner. While the engine is running, clean filtered air is drawn into the crankcase through the intake air filter and then through a hose leading to the oil filler cap or the valve cover. Early models covered in this product may have a filter here that requires maintenance. As the air passes through the crankcase it picks up the combustion gases and carries them out of the crankcase, up through the PCV valve and into the intake manifold. After they enter the intake manifold they are drawn into the combustion chamber and are burned.
The most critical component of the system is the PCV valve. This vacuum-controlled valve regulates the amount of gases that 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.
Fig. Schematic of a typical PCV system
Fig. PCV valve position with no vapor flow-engine off or pop-back
Fig. PCV valve position with minimal vapor flow-high intake manifold vacuum
Fig. PCV valve position with maximum vapor flow-moderate intake manifold vacuum
A molded vacuum tube connects a fitting on the intake manifold to a fixed orifice fitting of a calibrated size. This fitting meters the amount of crankcase vapors drawn out of the engine. The fixed orifice fitting is located on the side of the valve cover on 2.5L engines, the right valve cover on 8.0L engines.
A fresh air supply hose from the air cleaner housing is connected to a fitting at the top/rear of cylinder head cover on 2.5L engines, on the left valve cover on 8.0L engines.
When the engine is running, fresh air enters the engine through the fresh air supply hose and mixes with crankcase vapors. Engine manifold vacuum draws the vapor/air mixture through the fixed orifice and into the intake manifold. The vapors are then consumed during engine combustion.
Diagnosis & Testing
No maintenance is required. Check hoses for condition and replace if cracked or abraded.
On the 8.0L engine, only the right valve cover hose has a fixed orifice in the system (gray in color). The left side (fresh air) hose has a black connector. Do NOT interchange left and right side hose connections, as this would restrict fresh air supply.
Fig. CCV fixed orifice and hose-8.0L engine
- Some early models may be fitted with a crankcase inlet air cleaner on the valve cover (the valve cover opposite the one with the PCV valve, if there are two). This supplies make-up air to the engine to replace that vented through the PCV valve.
- Wash the valve in kerosene and dry it thoroughly before refitting.
- Maintenance should be performed more often if the vehicle is used for short trips, extended idling periods or very dusty conditions.
- Check the breather hose for restrictions.
- Check the intake manifold fittings for sludge buildup, this can reduce the flow of the system.
Fig. On the 4.7L, the PCV valve is in the oil filler: twist and pull to remove
Fig. On other models the PCV valve fits into a rubber grommet on the valve cover; replace the grommet if it leaks
Fig. Testing a PCV valve: vacuum should be strong and steady
Fig. Checking PCV suction with a piece of paper over the fresh air inlet
Fig. The crankcase inlet air cleaner (early models) should be cleaned in kerosene
- The PCV valve is located in the valve cover on all engines except the 4.7L, where it is located on the oil filler tube housing.
- With the engine running, pull the PCV valve and hose from the valve rocker cover rubber grommet or oil filler housing on the 4.7L. On this engine, twist it 90- and then pull it out.
- 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, 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. If not operating properly, replace the valve.
- If no vacuum is felt at the PCV valve when it is removed from the engine, remove the valve from the hose and check the vacuum supply in the hose.
- To check the PCV valve in operation, remove the fresh air fitting from the valve cover. Place a stiff piece of paper over the fitting grommet. After about a minute, the paper should be drawn against the grommet with considerable force. If not, replace the PCV valve.
Removal & Installation
- The PCV valve can be found on the valve cover, except on 4.7 L engines which have it on the oil filler tube.
- To remove a valve cover PCV valve, simply pull it out of the rubber grommet. Disconnect the hose, if required.
- To remove oil filler PCV valve, first disconnect the hose. Rotate the valve CCW until the locating tabs have been freed at cam lock, and then, pull the valve straight out.
Fig. Some models have the PCV valve located in the oil fill tube. This PCV valve must be rotated counterclockwise before pulling it out-4.7L engine shown
Fig. Pull the PCV valve out from the rubber grommet
- See "DRIVEABILITY AND EMISSION CONTROLS" for PCV valve tests.
- Inspect the inside of the hose. If it is dirty, disconnect it from the intake manifold and clean it with a safe solvent.
Fig. Separate the PCV valve from the hose
Fig. The hoses used in the crankcase ventilation systems are just as important as the valves. Be sure to inspect the entire system whenever a problem is evident-4.7L engine shown
To install:
- If the PCV valve hose was removed, connect it to the intake manifold.
- Connect the PCV valve to its hose.
- Install the PCV valve into the rubber grommet.
Refer to "GENERAL INFORMATION AND MAINTENANCE" section for removal and installation of the PCV valve.
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