BMW E39 crankcase breather freezes in winter
Why does the E39 crankcase breather freeze in winter?
Moisture condenses inside the breather pipe when the inside is warm and the outside is in frost. The moisture freezes and blocks the pipe, so the crankcase ventilation does not work and oil can get out. The phenomenon is common on M52 and M54 engines.
Why is there mayonnaise under the oil cap?
Crankcase gas condenses on a cold surface and water moisture mixes into it. Short trips in hard frost and damp weather make it worse. A blocked breather makes it even worse.
How is freezing prevented?
The solution is an insulated winter breather kit. The parts are cheap, but replacing them is fairly laborious, and the plastics are brittle. The M54 separator fits the M52TU engine, and the hoses are replaced at the same time. The blockage can also be at the lower end of the dipstick tube, and it can be cleaned.
How do I recognise a blocked breather?
With the engine running, when you remove the cap of the oil filler opening it should feel like moderate suction. If the suction noise is strong and the cap is hard to remove, the oil separator should be replaced.