An engine must breathe, and we tend to associate good breathing with an efficient, clean air filter and an unobstructed well designed air intake system.
The fact that the crankcase must also breathe is often over looked. We often here of engines which blow oil passed the dipstick but show no evidence of excessive fumes passing the crankcase breather.
A relatively small amount of pressure from the combustion spaces will in evitably pass the piston rings particularly during the running in period; normally this very small amount of pressure will be gently ventilated through the crankcase breather system (PCV) and the engine will function correctly.
If the breathing system should become blocked or restricted, the crankcase can become pressurised, if this occurs the pressure will reduce the efficiency of the oil control rings and problems with high oil consumption may be caused. Poor crankcase ventilation will also prevent the engine disposing of impurities such as water vapour and acids which are formed as a by product of combustion; these will reduce the life expectancy of the lubricating oil and oil seals, causing sludging and engine operating problems. The build up of gases in a pressurised crank case will attempt to ventilate wherever possible, such as dip stick, rings, seals guides etc.
A small amount of time spent ensuring that the crankcase breathe system (valves, pipes, tappet cavers, etc) is clean and that the system is free to operate, will save a large amount ot time investigating and curing the problems that a blocked breathing system may cause.