BMW 135i – Serpentine Belt Issues and Fix (N54)

A few weeks ago the car started making a squeaking noise. So, I popped the hood and started taking a video. Here is what I found:

As soon as I noticed the shredding serpentine belt, I drove home and parked the car. Ordered a new belt and tensioner, along with oil filter housing gaskets (#1 & #2) because mine were leaking. Once the parts arrived, Bob helped me install everything. Here is a picture of the OFH gaskets going in, what a mess…
Oil filter housing gaskets

Obligatory picture of the shredded belt:
Shredded serpentine beltWe put everything back together and assumed all was good. Wrong. About 50 miles later, the belt started shredding again. This time it started wrapping around the alternator pulley and burnt up the backside of the brand new tensioner pulleys.

Belt wrapped around alt pulley Burned tensioner pulleysSomething was clearly causing the problem. Unsure where to start, we looked at the alternator because the belt wrapped around that specific pulley. The alternator pulley has a sprag clutch inside that allows it to spin one way, but not the other. This video shows how to test yours. After watching the video, I went out and tested mine. I carefully lodged a screwdriver into the fan to lock it from spinning:
Locking the alternator pulleyOnce the alternator was locked, I tried spinning the pulley by hand in either direction. It was seized and would not move. So, I assumed that was the problem. This time I ordered another new belt, tensioner, as well as an upper idler pulley, a lower idler pulley, an alternator pulley, and an alternator pulley removal tool.

All of the parts arrived and we installed everything for a second time. The new alternator pulley spun smoothly in one direction as it was supposed to. I thought everything went great and drove the car around for 2 days until the belt shredded AGAIN! This time the belt started tearing and worked itself off of the pulleys. I was shocked.

Serpentine belt issues

I went online and starting reading up on the repeating issue. I saw a lot of discussion about the power steering (PS) pump pulley coming into contact with the subframe. The pulley would hit the subframe and shred the belt or break the pulley. This was caused by worn motor mounts, allowing the motor to flex and sag when accelerating.

However, this issue only affected N54 cars produced up until 04/2008. Apparently BMW addressed the issue at that point. My car was built in 08/2008 so I didn’t think it was my issue, but I took a look anyways. While underneath the car, I noticed that this was NOT my issue. My PS pulley was NOT coming in contact with the subframe. But upon further inspection, this is what I noticed:

1 of my PS pump bolts was missing and the other 2 were loose! This is not on the pulley, but on the pump itself. After doing a little more research, a few others reported the same thing. Maybe it’s because the bolts are attached to the oil pan, become coated in oil, and eventually work themselves loose… I’m not sure.

Either way, I removed the 2 loose bolts and found another bolt that fit perfectly in the missing slot. I added a drop of thread-locker to each bolt and reinserted them. I couldn’t find the torque specs, but I torqued them down to 40 ft/lbs.

This time around the tensioner and all pulleys were still in good shape, so I just bought another belt (for the 3rd time). I installed the belt and everything has been running smooth… fingers crossed.

As you can see, the engine bay was pretty dirty after all of this. So, this gave me an excuse to clean it up. No more dirt and grime!
Cleaned Engine Bay

Important Notes:

  • Check all pulleys the first time around! Make sure there is no play or movement in any of them. They should all spin freely.
  • If your N54 was produced before 04/2008, check to see if your PS pulley is coming in contact with your subframe. I’ve read that you can install 335is motor mounts (passenger side, driver side) to fix the problem. You can also notch the subframe.
  • If your belt has started to shred, pull over immediately! If the belt works its way around the crankshaft pulley, it can break through the front main seal and even work its way into the timing belt. Best case scenario: the main seal needs to be replaced (oil leak). Even worse, the main seal continues to leak and you run the motor dry. Much worse, the belt causes the motor to jump timing and self-implode. No one wants that, so be sure to check behind the crankshaft pulley for belt fragments. If the belt wrapped around the backside of the pulley, remove it and carefully inspect the front main seal.

22 thoughts on “BMW 135i – Serpentine Belt Issues and Fix (N54)”

    1. Sprayed the whole bay down with Purple Power degreaser. Then I covered the important stuff (intake filters, ECU, alternator, etc) and power washed it all down. Came out fantastic!

  1. Thanks for the write up. What is the easiest way to get a look at the bolts? I tried to take a look from under the hood, but it seemed pretty deep in there.

    1. Joe,

      Sorry you’re having troubles. The easiest way to access these bolts is to lift the car up, then slide under the front end. If you’re on your back, you should be able to look up and grab the PS pump and check the bolts.

  2. I really appreciate your having made this page. You consolidated 5 possible issues, so I was able to rectify them all. In the process if addressing all these potential problems. Had 2 pulleys shred on me. No front main leak, fortunately. Fyi, my 335i was manufactured in 07. Had a missing PS pump bolt and a siezed Alt pulley upon inspection. Given that I have a sizable amount put into such a nice car, what’s it worth to protect the asset? Definitely a few pulleys and engine mounts.
    Thanks again for the peace of mind.

  3. I accidentally dropped a bolt inside the plastic pipe that runs in-front of the serpentine belt and leeds to the intake filter. Did you guys remove that pipe when installing the serpentine belt? I’ve tried looking where it goes under the motor but cannot see a connection, Im trying to disconnect that pipe so I can remove the bolt. Im scared to drive the car, and the bolt end up somewhere or cause damage. Any Ideas?

    1. That pipe connects to the front turbo. It’s not very easy to disconnect, but can be done with the fan removed, etc. Otherwise you can try snaking a magnet down the pipe.

      Do not start the car! The bolt will get sucked into the turbo and destroy it.

  4. Hi Jake,

    Everything you said happened to me today except all my oil leaked out while I was in school in the parking lot. I only drove the car for 3 min after the serpentine belt shredded which was before the oil leakage. When I came out for a break I noticed large new oil spot on the ground coming from my car so I got a tow truck to take me to the mechanic. however when I got to the shop I was told they had to make sure my engine isn’t damaged due to no oil . What do you think if all the oil came out at my school and I didn’t drive after that. Do you think it will be ok?

    1. Sorry that you’re having the same issues.

      If your car is leaking oil, pieces of the belt probably got sucked past the front main seal and got into the motor. I would definitely get it checked out. You will have to remove the pieces from the engine/oil pan and address the front seal to stop the leak.

  5. Hi there, thanks so much for this thread! I have a 2009 135i and today when I was almost 1mile away from the road, the charge warning came on and I lost the power steering, I was in the middle of a 4 lane highway and luckily in 3 mins I was home at which point I saw the shredded belt. There’s no oil leak, engine sounded fine – I will show this thread to my mechanic so he can go through these checks, any advice that I can check in the meantime tomorrow to give me some peace of mind? Also, any other issues as such experienced since? Thanks a lot!

    1. Hello, sorry for the late reply! I haven’t logged on in a few weeks. Hopefully you’ve managed to fix your problem. If there isn’t any leaking oil and the engine sounds normal, I’d assume everything is good. I haven’t had any issues since tightening the PS pump.

  6. Hi jake, I stumbled on this thread and I thank you for this info. Yesterday my serpentine belt shredded. The car was parked at the time and I just started it up before leaving work. I removed the shredded belt started the car again and it sounds normal but I’m nervous hearing that pieces could get sucked into crankcase. Seeing that the engine sounds fine and no oil leak do you think it will be alright. Thanks

    1. Hey Greg, thanks for commenting! Sucks about the belt. If your engine sounds fine and you don’t see any oil leaks, I would assume you are okay. Keep an eye on that crankshaft area for leaks, but I wouldn’t worry about it.

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