It is extremely important for both engine oil control and performance that the crankcase breathing system be clean and functional at all times. A clogged or inoperative crankcase ventilation system will lead to poor engine performance, rapidly wear out rings and cylinder bores, stick rings, valve lifters, valves, and cause sludge formations which can clog oil passages throughout the engine.

Most recent engines are equipped with a positive ventilation system, which means that the engine is a sealed unit as far as crankcase fumes and pressures are concerned. The fumes, blow-by, and other crankcase by-products are recirculated through the fuel intake system, burned with the fuel, and subsequently expelled through the exhaust system.

Older engines are equipped with a ventilated oil filter cap which allows fresh air to be drawn into the crankcase. There is a partial vacuum induced in the crankcase by having a breather pipe extending toward the bottom of the engine, which by the shape of the pipe opening and the movement of the vehicle, a low pressure area is created to draw the fumes, blow-by, and combustion by-products out of the oil.

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Prior to 1963 most vehicle engines vented their vapours and oil deposits to atmosphere and the road surface! With increasing environmental pressures Positive Crankshaft Ventilation was introduced whereby the crankcase vapours were drawn up into the inlet manifold and, along with the air/fuel mixture, burned up in the combustion chambers. To enable this system to work safely and efficiently the ventilation from the crankcase is controlled via a PCV valve.

To avoid upsetting the fuel/air mixture, the PCV valve must regulate the evacuation of these blow-by gases and vapours (which will be minimal at idling speed but will intensify as engine speed is increased). Since manifold vacuum is highest at low engine speeds, the PCV plunger will be drawn forward to a position that will restrict crankcase ventilation to a minimum thus ensuring no unsettlement of the air/fuel mixture. As engine speeds are increased the manifold vacuum will drop thus reducing the ‘pull’ on the plunger which will slide back to a midway position allowing a greater flow rate from the crankcase. Since the engine demands more fuel/air mixture at high engine speeds, the escalation of crankcase vapours into the combustion chambers should not affect performance.

The PCV valve also acts as a flame trap. In the event of a backfire, the resulting pressure through the inlet manifold will force the plunger back into the closed position, thus preventing an explosion of the vapours in the crankcase. Various PCV systems are in use but they all function in essentially the same way. Earlier systems were known as ‘open’ systems that still allowed some vapours to vent to atmosphere via the filler cap. ‘Closed’ PCV systems have been the norm for some time now, whereby the filler caps are not vented and air is recirculated via the air filter. Left unchecked over a period of time a PCV system will deteriorate and may cause major engine problems as outlined above. Regular maintenance is essential with some manufacturers recommending the renewal of the PCV valve at every major service interval.


The positive type of system utilizes manifold vacuum to draw the fumes out of the crankcase. The amount of vacuum is controlled by a positive crankcase ventilation (PCV) valve. This valve, when inoperative, will lead to either oil consumed through the intake system when stuck in the open position or when stuck in the closed position will lead to oil blowing back out the filter cap and out past the engine seals.

The consequences are exactly the same on the older standard type of system when the breather pipe, or breather cap are not clean. Some engines utilizing this system have a screen through which the breather pipe draws fumes. This also can become plugged and cause the previously mentioned problems.

Most positive systems recommend cleaning every 6,000 miles and valve replacement every 12,000 miles. The standard type usually recommend cleaning when oil is changed. In either case, the breathing system should be completely gone through when major piston ring work is being performed. The success of a piston ring job today is very much dependent upon the engine being able to breathe properly.