Ford Mid-Size Cars 1971-1985 Repair Guide

Positive Crankcase Ventilation System


See Figures 1, 2 and 3

All models are equipped with a Positive Crankcase Ventilation (PCV) system to control crankcase blow-by vapors. The system consists of a PCV valve and oil separator mounted on top of the valve cover, a non-ventilated oil filter cap, and a pair of hoses supplying filtered intake air to the valve cover and delivering the crankcase vapors from the valve cover to the intake manifold (6-cylinder) or carburetor (V8). 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 in the crankcase. 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 these gases back into the engine combustion chamber where they are reburned.

As the engine is running, clean, filtered air is drawn through the air filter and into the crankcase. As the air passes through the crankcase, it picks up the combustion gases and carries them out of the crankcase, through the oil separator, through the PCV valve, and into the induction system. As they enter the intake manifold, they are drawn into the combustion chamber where they are reburned.

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Fig. Fig. 1: PCV circulation through the engine

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Fig. Fig. 2: Standard PCV valve installation in a V8 engine

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Fig. Fig. 3: PCV valve operation

The most critical component in the system is the PCV valve. This valve controls the amount of gases which are recycled into the combustion chamber. At low engine speeds, the valve is partially closed, limiting the flow of the 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 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 4

With a Tachometer
  1. See if any deposits are present in the carburetor passages, the oil filler cap or the hoses. Clean these are required.
  3. Connect a tachometer, as instructed by its manufacturer, to the engine.
  5. With engine idling, do one of the following:

If the valve and the hoses are not clogged up, a hissing sound should be present.

  1. Check the tachometer reading. Place a finger over the valve or hose opening (a suction should be felt).
  3. Check the tachometer again. The engine speed should have dropped at least 30 rpm. It should return to normal when the finger is removed from the opening.
  5. If the engine does not change speed or if the change is less than 30 rpm, the hose is clogged or the valve is defective. Check the hose first. If the hose is not clogged, replace, do not attempt to repair, the PCV valve.
  7. Test the new valve in the above manner, to make sure that it is operating properly.

Without a Tachometer

With the engine running, pull the PCV valve and hose from the valve rocker cover rubber grommet. Block off the end of the valve with your finger. A strong vacuum should be felt. Shake the valve; a clicking noise indicates it is free. Replace the valve if it is suspected of being blocked.


  1. Pull the PCV valve and hose from the rubber grommet in the rocker arm cover or from the oil filler cap.
  3. Remove the PCV valve from the hose. Inspect the inside of the PCV valve. If the valve is gummy it can be cleaned in a suitable, safe solvent. However, replacing a clogged, gummed up PCV valve with a new one is suggested.
  5. Soak the rubber ventilation hose(s) in a low volatility petroleum base solvent to loosen the deposits. Pass a suitable cleaning brush through them and blow out with compressed air or let air-dry.
  7. Thoroughly wash the crankcase breather cap (if equipped) in solvent and shake dry. Do not dry with compressed air; damage to the filtering material may result.
  9. Replace any hard or cracked hoses or ones that are clogged and cannot be cleaned.
  11. The installation of the hoses and PCV valve is in the reverse order of removal.

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Fig. Fig. 4: PCV diagnostic chart