Ford Ranger/Bronco II 1983-1990 Repair Guide

Crankcase Emission Controls



See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Exploded view of a common PCV system

The crankcase emission control equipment consists of a Positive Crankcase Ventilation (PCV) valve, a closed oil filler cap and the hoses that connect this equipment.

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 were 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.

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 are burned.

The most critical component of the system is the PCV valve. This vacuum-controlled valve regulates the amount of gases which are recycled into 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 gasses 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.


See Figure 2

  1. With the engine running, pull the PCV valve and hose from the valve rocker cover rubber grommet.
  3. Block off the end of the valve with your finger. A strong vacuum should be felt.
  5. Turn the engine OFF .
  7. Shake the valve; a clicking noise indicates it is free.
  9. Replace the valve if it is suspected of being blocked or either of the two tests above did not yield the proper results.

Replace the PCV valve instead of trying to clean it. Also, check the PCV valve hose for cracks or wear.

Click image to see an enlarged view

Fig. Fig. 2: Check the PCV valve for vacuum at idle


See Figure 3

Click image to see an enlarged view

Fig. Fig. 3: To remove the PCV valve, pull it free from the valve cover grommet then disconnect the vacuum hose from the 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.