The 3.9L, 4.7L, 5.2L and 5.9L gasoline engines are fitted with a Positive Crankcase Ventilation (PCV) system.
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 guide 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.
- 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 90º 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.
- 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.
REMOVAL & INSTALLATION
Refer to "GENERAL INFORMATION AND MAINTENANCE" section for removal and installation of the PCV valve.