[defurr]

My 2011 335i coupe with FMIC and Cobb Aggressive tune just recently lost its serpentine belt, and I would love to understand why this happened. So any theories on the following would be welcomed.

Of course, when the belt came off, it went into the engine through the crank shaft seal. In addition to cleaning belt shred out of the engine, the repair included resetting the timing, replacing both Vanos solenoids, and replacing the boost solenoid.

The only credible explanation I’ve received from my indy is that he saw more oil on the lower extremities of the block than he would have expected, leading him to believe that the crank shaft seal was leaking oil onto the belt. He says that the tensioner pulley (which he himself had replaced a year ago) seemed fine. And he had also replaced the oil filter housing gasket along with the oil-soaked serpentine belt and alternator four months ago.

After the oil filter housing gasket repair, I had added the FMIC and bumped the tune up to Stage 1 + FMIC Aggressive on the Cobb. Soon thereafter, my engine began to throw a CEL when revving from low RPMs at low torque. It would happen once every few days, but never seemed to run poorly as a result-- always clearing between drives. I would check the OBDs and see random misfire codes like coolant temp sensor (P2EE2) and electric fans (P2EFF) that seem like red herrings.

Only other clue is that the serpentine was squeaking at me during cold temps right after I got it back from the oil filter housing gasket. It would chirp (barely audible inside the car) until the engine warmed up, and then it would sound normal.

[Kevin39]

I would have guessed the oil filter housing as well. Maybe someone else has a better guess.

[JETmn]

Squeaking belt usually means it is due for a replacement or a pulley is going bad. Was it replaced during the OFHG replacement? If not, it should have been.

[lwgrenier]

There are a couple pulleys other than the tensioner pulley he should have replaced. However even if they're squeaking I can't see them being a reason for a snapped or slipping belt.

[Pladi]

The thing here is you dont know that thje belt snapped or broke as you described on your title. The belt most likely slipped as a result of oil residue still there from the previous OFHG leak.

[biz77]

Are you sure the chirping noise you heard was the serpentine belt and not the front main seal?