Cause
Frozen crankcase breather in cold weather
The breather blocked by ice rather than sludge. Crankcase gas carries a great deal of water vapour, and in freezing weather it condenses and freezes in the breather passage — most readily on an engine doing short journeys that never boil the moisture off.
The effect is the same as any blockage and arrives suddenly: crankcase pressure with nowhere to go, and oil forced past a seal. On a badly frozen system the pressure can be enough to blow a dipstick out or push the cam cover gasket, which is dramatic and entirely seasonal. Many engines carry a heated breather for exactly this, so check whether one is fitted and whether its heater works. The underlying cause is usage — short cold journeys — which is a genuine finding for the owner rather than a part.
Typically an easy fix. Often worth checking before paying anyone to look.
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