Ford Mustang/Capri 1979-1988 Repair Guide

Positive Crankcase Ventilation System


See Figure 1

All models covered in this guide are equipped with a Positive Crankcase Ventilation (PCV) system to control crankcase blow-by vapors. The system consists of a PCV valve, a non-ventilated oil filler cap, an oil separator and a pair of hoses that supply filtered intake air to the rocker arm cover, while delivering crankcase vapors from the rocker arm cover to the intake manifold, carburetor or throttle body. The PCV valve is often mounted in a grommet on top of the rocker arm cover. On engines equipped with an externally mounted oil separator, the PCV valve may instead be located between a vent hose and a manifold vacuum source. The system functions as follows:

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 the atmosphere. If these gases are allowed to remain in the crankcase for any period of time, they contaminate the engine oil and cause sludge to build up. If the gases are allowed to escape into the atmosphere, they pollute the air with unburned hydrocarbons. The job of the crankcase emission control equipment is to recycle the gases back into the engine combustion chamber where they are burned.

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Fig. Fig. 1: PCV system operation-V8 engine shown


  1. Remove the PCV valve from the rocker arm cover grommet or other underhood location.
  3. Shake the PCV valve. If the valve rattles when shaken, reinstall and proceed to Step 3. If the valve does not rattle, it is sticking and must be replaced.
  5. With the engine at idle, disconnect the hose from the air cleaner and feel for vacuum at the hose. Some vacuum indicates that the system is okay. If there is no vacuum, proceed to Step 4.
  7. No vacuum indicates that either the system is plugged or the evaporative valve is leaking. Proceed as follows:
    1. Disconnect the evaporative hose, cap the tee and retest. Some vacuum indicates that the PCV system is okay. Check the evaporative system.
    3. If there is still no vacuum, check for vacuum back through the system (oil filler cap, PCV valve, hoses and oil separator). Also check the rocker arm cover(s) for correct bolt torque and/or a gasket leak. Service as necessary.



Since the PCV valve works under severe load, it is very important that it be replaced at the interval specified in the maintenance chart (see General Information & Maintenance ).

On many engines, the PCV valve is mounted in a grommet in the rocker arm cover. On 4-140 engines equipped with an externally mounted oil separator, the PCV valve is positioned between two or more hoses near the intake manifold.

Remove the PCV valve from the grommet in the rocker arm cover, if applicable.
Disconnect the hose(s) from the valve. Do not attempt to clean a used valve.

To install:
  1. Connect the hose(s) to the new PCV valve.
  3. Insert valve into the rocker arm grommet, if applicable.