To discourage oil from being thrown into the combustion chambers and to prevent any chance of fire, modern crankcase ventilation systems incorporate an oil separator and a valve or flame trap.
In time, these restrictors can become clogged with deposits carried by the oily fumes, and if they are not unblocked, the resulting pressure build-up causes oil leaks. Most car makers recommend cleaning the breather assembly at 10,000-12,000-mile intervals. As most cars now have a closed or semi-closed breathing system which includes a spring-loaded ventilation valve, these types are dealt with here.
Remove the oil filler cap. If it has ventilation holes and contains a gauze filter, rinse it in paraffin and shake off the surplus before refitting it.
On caps without ventilation holes, check that the rubber sealing ring is sound and able to make an airtight joint. If the ring is damaged, fit a new cap.
Locate the ventilation valve, on most cars it is at one end of the ventilation hose joining the crankcase to the inlet manifold or carburetor air cleaner.
Remove the valve assembly and clean it with an old paint brush dipped in paraffin. Test its action by gently pressing the valve into the housing; it should move easily and freely against slight spring pressure. If it has a sticky action that cannot be improved by cleaning, fit a new valve. This may not be necessary as some valves can be dismantled after removing a circlip at the end.
Disc-shaped valves, mounted on the inlet manifold, must also be dismantled for cleaning. These valves have a spring-loaded diaphragm inside, and a top cover held by a spring clip. Move the clip aside, remove the cover and lift out the diaphragm and spring underneath. Clean all parts and reassemble in reverse order.
Undo the hoses and check them for blockages. To clean a sludged-up hose, make a pull-through by hooking the end of a length of stiff wire round a small piece of cloth. Thread the wire through the hose and use it to draw through the cloth.
On reassembly, make sure that all hoses fit firmly and make an airtight joint. Air leaks cause erratic engine idling.