What could cause this belt wear?

[ps2cho]

The tensioner & pulley, both idler pulleys are all damaged. Tensioner particularly. The tensioner pulley spins totally fine, theres maybe a very small amount of play (compared to the brand new BMW one I just got) but nothing that jumps out as an obvious culprit.

This is only 13,900 miles on my 2021 M2C (S55 engine).
I've never seen something like this happen before. What the heck could have caused this?

I obviously will be replacing everything, tensioner pulleys and a new belt. But I want to be sure if you all think anything else could cause this other than a bad tensioner?

 

[thastinger]

I would guess that the chewed up tensioner isn't sitting true. I've had to put washers on 1 or 2 of 3 tensioner studs before so the tensioner and belt ride flush together.

 

[Cujet]

Misalignment of some sort is the answer. Heat builds up in the belt and the drama starts.

 

[grasscutter]

Looking at the wear pattern of the tensioner pulley in your picture, I'd say you have a misalignment issue.

 

[97prizm]

Looks like something got hot on the tensioner.

 

[ps2cho]

Looking at the wear pattern of the tensioner pulley in your picture, I'd say you have a misalignment issue.

I mean there isnt any adjustment or alignment. Its all fixed positions and the tensioner is spring loaded...

 

[TnT_M]

I mean there isnt any adjustment or alignment. Its all fixed positions and the tensioner is spring loaded...

put a straight edge across all the variety of pulleys. This clearly looks like something is out of alignment. Just because there is no adjustment doesn't mean something isn't out of alignment. Bad parts do get made in manufacturing and unfortunately some slip out the door.

 

[Hirev]

Its all fixed positions and the tensioner is spring loaded

Could be the part was not machined correctly.

 

[Arc]

I would bet on the problem being the belt. One of the metal strands was tighter than the rest causing unequal pressure on the plastic pulley.

 

[JHZR2]

Unfortunately this sort of thing is common in n-series engines. Some have attributed it to misaligned PS or AC pulleys, others to soft engine mounts allowing contact in certain conditions.

Here’s some reading:

https://www.e90post.com/forums/showthread.php?t=1514899

https://www.1addicts.com/forums/showthread.php?t=1462063

Consequences can be severe, ingestion of best through front main seal.

That said, I’ve seen a ridge on my ML320 which has a metal tensioner pulley. So part of me wonders if it’s because belts are getting bigger without the supporting hardware doing so.

 

[Marcozi]

Were those the original pulleys and belt?

 

[The_Eric]

Yikes! Looks like lots of heat and/or load. I wonder if it's possibly over-tensioned?

 

[xspeedy]

I wasn’t aware of BMWs ingesting belts until now. I did some searching, and it appears there are guards you can install to prevent this from happening. Probably something I would consider installing if I had a susceptible engine.

 

[slybunda]

if one of the cords inside the belt has snapped then it can put load on the belt in one side of it. maybe a stone got in there and caused belt damage which in turn ended up with this lop sided wear?