Accessories belt keeps falling off track

I just changed my idler pulley since it was squealing. I took it for a small drive to make sure everything was good, and then the belt shredded, not all the way but was about half the width it was before. So I buy another one, install it and a few minutes after start up I look at where it is seated on the idler pulley and it seems a little off. I put the car on ramps to check from the bottom and sure enough, it's hanging off the crank pulley, almost sitting on the other belts (this is the PS belts). So since it's not shredded, just a little & I mean barely miss sharpen from sitting on the outside of the crank, I seat it correctly and drive it fine for the day. No issues.

Now I start my car up today and check again like I always do after I fix something and what do you know, it's out of place again. I looked under the car while it was on and saw the idler pulley/tensioner was bouncing up and down. Idk if that is normal... but if anyone could help me out. Could it be since the belt was distorted a bit on the end from sitting on the crank pulley it just won't sit right anymore since it's not perfect flat on the edge or do I need a brand new tensioner?

I'm confused. What parts did you change? The pulley, tensioner, belt, or all three? I'm not sure what BMW recommends, but on my old Audi you were supposed to replace all three at the same time.

Remember that belts stretch as they age, but the tensioner should manage that.

dparm wrote

I'm confused. What parts did you change? The pulley, tensioner, belt, or all three? I'm not sure what BMW recommends, but on my old Audi you were supposed to replace all three at the same time.

Remember that belts stretch as they age, but the tensioner should manage that.

I only changed the idler pulley. The belts and under drive pulleys were changed when I upgraded to MS pulleys.

did you get the right pulley kit? the 08's are different from the 09's and on. not sure what issues you would have if you put on the wrong kit/belt, but check on that since that was the last thing you replaced.

roastbeef wrote

did you get the right pulley kit? the 08's are different from the 09's and on. not sure what issues you would have if you put on the wrong kit/belt, but check on that since that was the last thing you replaced.

It's the right kit. I've had the kit for 3 months now with no issues. The tensioner is no longer "bouncing" now that I seated the belt correctly again, and is still pretty strong when I push down on it. I'm thinking it's the belt since it was bent on the side a little causing it to be more prone to slip off the side... But still not sure. I'll just monitor it the next few days. Good thing is, the belt is warrantied.

My guess would be incorrect belt routing. I doubt two belts were defective. I think there are two versions of belt routing for these cars.

Theodore wrote

What is your car's build date? As mentioned already, the idler pulley and the belt routing are different for build dates before 4/29/08 vs. build dates on or after that date. Same is true for the belt itself, it is a different part number and slightly different size. Is it possible your dealer or parts store gave you the belt for the wrong production date? Check out this thread here to double check everything is right for your build date.

12/07 build date. It's all correct parts & routing. I've rechecked the routing multiple times now. The only thing that's changed is the new idler pulley, which is also the correct part #. I made sure. If something like that was off I'd notice..

Today when I went to work this morning, I made sure the night before the belt was in place. Started her up and checked the engine bay, everything normal, correct location on the idler pulley and crank pulley (those are the places it slips over about a rib). I drive to work all fine. Get off work, start the car and check the belts again. It's now moved over about a rib length on the crank pulley and idler pulley towards the engine... tensioner bouncing up & down, again...

I'm starting to feel the belt was damaged from slipping over and riding on the crank pulley edge, stretching the outer part of the belt causing it move over during start up...

Now I guess when I have free time I'll take the belt off, examine it & hopefully find enough evidence to get it replaced with warranty at Oriellys.

If you think it's defective belts why not order an OEM BMW belt just in case that's the problem rather than bang your head against the wall with yet another replacement from OReilly

I get burned by the generic parts about 50% of the time. AutoZone stocks brake pad sensors for the same price as ECS and their warehouse is here in Chicago; I've made two separate trips back because the part listed for our cars doesn't fit.

Had the same problem with my old S2000 and one of my old Audis.

The stores are always cool about returning/exchanging. I suspect they know the catalog is not very accurate.

Richbot wrote

If you think it's defective belts why not order an OEM BMW belt just in case that's the problem rather than bang your head against the wall with yet another replacement from OReilly

I have the MS pulley kit, it's different belts than OEM. And I'm not saying the belt is faulty alone. I think since it was stretched on the edge that's what's causing the problem.
I'll be returning it today since it happened yet again this morning.
Update later.

Why is the tensioner pulley bouncing up and down?

pbonsalb wrote

Why is the tensioner pulley bouncing up and down?

Update
Belt broke while driving back home from work. Lost power steering & AC not a big deal. Just a pain at low speeds.
Found out it is the tensioner that needs replacement. Had a someone else start my car while I watched; the tensioner would move forward just a bit on start up causing the belt to get off track.

shakywarrior wrote

Update
Belt broke while driving back home from work. Lost power steering & AC not a big deal. Just a pain at low speeds.
Found out it is the tensioner that needs replacement. Had a someone else start my car while I watched; the tensioner would move forward just a bit on start up causing the belt to get off track.

Oh the tensioner pulley would bounce up and down only when the belt was out of place.

Would I need just the hydraulic tensioner or the roller pulley part as well...

I would replace only the pulley unless the tensioner was no longer providing tension. I have not messed around with this tensioner, though I have installed the UD pulley and removed and reinstalled belts then and when I did rod bearings. On some tensioners there is a spacer behind the pulley. If there is one and it went missing, it could cause issues like you have. Check realoem for the parts diagram to see if there is one. Whenever I have problems after doing work on a car, I suspect my work first.

Is there a DIY for the belt tensioner? I read something about tourque angling? Not sure if I need to do that or just tourque it normally to the ft/lb spec.