Mitsubishi Pick-ups and Montero 1983-1995 Repair Guide

Positive Crankcase Ventilation (PCV) System



A closed positive crankcase ventilation system is used on all Mitsubishi models. This system cycles incompletely burned fuel (which works its way past the piston rings into the crankcase) back into the intake manifold for reburning with the fuel/air mixture. The oil filler cap is sealed and the air is drawn from the top of the crankcase into the intake manifold through a valve with a variable orifice.

This valve (commonly known as the PCV valve) regulates the flow of air into the manifold according to the amount of manifold vacuum. When the throttle plates are open fairly wide, the valve opens fully. However, at idle speed, when the manifold vacuum is at maximum, the PCV valve reduces the flow in order not to unnecessarily affect the small volume of mixture passing into the engine.

During most driving conditions, manifold vacuum is high and all of the vapor from the crankcase, plus a small amount of fresh air, is drawn into the manifold via the PCV valve. At full throttle, the increase in the volume of blow-by and the decrease in manifold vacuum make the flow via the PCV valve inadequate. Under these conditions, excess vapors are drawn into the air cleaner and pass into the engine along with the fresh air.

A plugged valve or hose may cause a rough idle, stalling or low idle speed, oil leaks in the engine and/or sludging and oil deposits within the engine and air cleaner. A leaking valve or hose could cause an erratic idle or stalling.


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

Rough idle, not explained by an ordinary vacuum leak, 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.

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 from the valve cover, 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 the valve is a threaded type, it may be necessary to use a thin probe inserted in the threaded end to check for plunger motion. 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.


Remove the PCV valve from the valve cover. Most valves are pressed into the valve cover but some are threaded and screw into the cover. Remove the hose from the valve. Take note of which end of the valve was in the manifold. This one-way valve must be reinstalled correctly or it will not function. While the valve is removed, the hoses should be checked for splits, kinks and blockages. Check the vacuum port (that the hoses connect to) for any clogging.

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.