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The separator that removes oil mist from crankcase gases before they return to the intake, restricted so that the crankcase can no longer breathe. It is the part of the ventilation system most often overlooked, because the valve and the hoses are cheap and visible while the separator is usually integrated into the cam cover or the block.

Blocked, it pressurises the crankcase, and pressure finds the weakest seal. Oil is forced past the crankshaft seals, the dipstick tube or the cam cover gasket — so the complaint arrives as an oil leak, and replacing the seal without relieving the pressure means the new one leaks too. A leak that returns quickly after a competent repair is this fault, and it is the single most useful thing to know about it.

It also breaks the intake side. A separator that has failed the other way passes oil mist downstream, which coats the airflow sensor, fouls the throttle body and eventually reaches the intake valves. So one part explains both an external oil leak and a set of intake and metering complaints, and checking crankcase pressure at the oil filler is a quick test that settles it.