The brief
The Golf had been needing oil top-ups every other tank, the liner under the engine was getting greasy, and the idle had grown rough enough to feel through the wheel at the lights. Three pointers at the oil separator and the PCV system.
The oil separator is part of the engine's breather system. The engine breathes out a fine oil mist from inside, the separator catches that oil and drops it back into the sump, and the cleaned vapour goes back to the intake to be burned. A diaphragm inside the separator keeps the pressure in check while it does this. When the diaphragm fails and the housing cracks, that whole path stops working: crankcase pressure isn't regulated, which upsets the idle, and oil mist gets pulled straight through the breather instead of being caught and drained, which is the oil use and the mess.
The diagnosis
On the lift, the leak traced to the oil separator cover, which had a hairline crack near one of its mounting bosses. And the diaphragm inside the unit had failed too. With both gone, the engine was no longer holding crankcase pressure cleanly, which is why it idled rough and why it was drinking oil through the breather path.
That's a replacement. The separator is a sealed assembly, you don't patch a cracked housing or a torn diaphragm, so the unit gets changed.
The work
The cracked oil separator cover came off, and a new VAG-spec unit went on in its place with a fresh diaphragm and seal. The breather hoses were reseated with new clamps so the whole path was sealed again.
Then a short adaptation cycle was run on the scanner so the engine relearned its idle with the breather working properly.
A road test confirmed the idle had settled and there was nothing leaking.
The outcome
No more leak, a smooth idle, and oil consumption back to normal.
The Golf went home with the PCV system regulating cleanly and the bay liner dry. A failed oil separator quietly drinks oil and roughens the idle the longer it's run, so catching it now kept it to a clean part swap.