Ford Aerostar 1986-1997 Repair Guide

Positive Crankcase Ventilation (PCV) System



The Positive Crankcase Ventilation (PCV) system incorporated into the gasoline engine cycles crankcase gases back through the engine where they are burned. The PCV valve regulates the amount of ventilating air and blow-by gas to the intake manifold and also prevents backfire from traveling into the crankcase. The PCV valve should always be mounted in a vertical position. On some engine applications, the PCV system is connected with the evaporative emission system.


See Figure 1

If any of the following operating problems occur, inspect the PCV system:

Rough idle, not explained by an ordinary vacuum leakage, or fuel delivery problem.
Oil leaks past the valve cover, oil pan seals or even front and rear crankshaft seals not explainable by age, high mileage or lack of basic maintenance.
Excessive dirtiness of the air cleaner cartridge at low mileage.
Noticeable dirtiness in the engine oil due to fuel dilution well before normal oil change interval.

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Fig. Fig. 1: Check the PCV valve for vacuum at idle

An engine with badly worn piston rings and/or valve seals may produce so much blow-by that even a normally functioning PCV system cannot deal with it. A compression test should be performed if extreme wear is suspected.

The PCV system is easily checked with the engine running at normal idle speed (warmed up). Remove the PCV valve, but leave it connected to its hose. Place your thumb over the end of the valve to check for vacuum. If there is no vacuum, check for plugged hoses or ports. If these are open, the valve is faulty. With the engine OFF , remove the PCV valve completely. Shake it end to end, listening for the rattle of the needle inside the valve. Generally, if no rattle is heard, the needle is jammed (probably with oil sludge) and the valve should be replaced. If no motion is felt, replace the valve.

An engine which is operated without crankcase ventilation can be damaged very quickly. It is important to check and change the PCV valve at regular maintenance intervals.


See Figures 2, 3 and 4

Do not attempt to clean an old PCV valve using solvent or any other cleaning material. If the valve is suspect, it should be replaced.

Remove the PCV valve from the valve cover. Depending on the model year, some valves are pressed into the valve cover and others are threaded into the cover. Remove the hose from the valve. Take note of which end of the valve installed. This one-way valve must be reinstalled correctly or it will not function correctly. While the valve is removed, the hoses should be checked for splits, kinks and blockages.

Remember that the correct function of the PCV system is based on a sealed engine. An air leak at the oil filler cap and/or around the oil pan can defeat the design of the system.

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Fig. Fig. 2: PCV valve-2.3L engine

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Fig. Fig. 3: PCV valve-3.0L engine

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Fig. Fig. 4: PCV valve-4.0L engine