[bung206]

The serpentine belt in my 2011 N55 equipped 135i with 38000 miles tore off the other day.

There was no real damage to the belt at all, no contaminants on any of the pulleys. I didn't see any oil leaks on the engine (under tray was clean). I got no warning of any kind (no squeaking etc) - it just tore off. The only indication something was wrong was a slapping sound (which was the belt smacking on various things in the engine bay). I noticed it just as I pulled into my garage.

When I took off the plastic intake bit at the front and surveyed the damage, a large percentage (maybe 60%) of the belt was missing and whatever remained was holding on by a thread. Significant portions of the belt have gotten sucked into the fan as well, they were wrapped around the blades but luckily nothing broke. Quite a bit of the belt had wrapped itself around the alternator pulley.

When I pulled out the radiator fan and removed the under body protection, there were pieces of belt everywhere in the engine bay (both rubber an thread).

I've since replaced the belt and tensioner, everything seems fine now. The fan runs without issues as well (I was seriously concerned it had become un-balanced since it took a beating when the belt was thrown into it.

I inspected the broken belt very closely. The sections of it that are intact appear to be in very good condition - the belt isn't compromised or cracked or anything.

Just wondering what could be the cause of this.. I'd hate for this to happen again and strand me or worse... destroy the front main seal.