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.

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.

(P36-37)