Jeep CJ/Scrambler 1971-1986 Repair Guide

Crankcase Emission Controls


See Figure 1


The crankcase emission control equipment consists of a positive crankcase ventilation valve (PCV), a closed or open oil filler cap and hoses to connect this equipment.

When the engine is running, a small portion of the gases which are formed in the combustion chamber during combustion, leak by the piston rings and enter the crankcase. Since these gases are under pressure, they tend to escape from the crankcase and enter the atmosphere. If these gases were allowed to remain in the 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 either directly through the oil filler cap, or through the carburetor air filter and then through a hose leading to the oil filler cap. 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 burned.

The most critical component in the system 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 from the crankcases 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 above explanation and the troubleshooting procedure which follows applies to all engines with PCV systems.

Click image to see an enlarged view

Fig. Fig. 1: PCV system as seen on most engines


With the engine running, pull the PCV valve and hose from the engine. Block off the end of the valve with your finger. The engine speed should drop at least 50 rpm when the end of the valve is blocked. If the engine speed does not drop at least 50 rpm, then the valve is defective and should be replaced.


  1. Pull the PCV valve and hose from the engine.
  3. Remove the PCV valve from the hose. Inspect the inside of the PCV valve from the hose. If it is dirty, disconnect if from the intake manifold and clean it.