Stumped. AC belt keeps slipping off

[ThreeD]

It might be due to the higher rpm and maybe ur ltw flywheel. Do u get rattling at 700 or 750rpm idle?
INA is one of THE oe brands. I trust it like I trust lemforder. The problem has got to be something else. Is ur crank tensioner old?

That's what I thought too regarding INA. I do not experience significant gear rattle at 700-750 rpm.

With the higher redline I thought it could be an issue, but there are so many people running ltw flywheel with raised redlines at autocross / track events, bouncing off the limiter all day without AC issues. I'd find it hard to believe that solely LWFW and higher redline are causing this.

What crank tensioner are you referring to? The crank pulley / harmonic balancer itself is as old as the car is, ~140k miles.

[pbonsalb]

I had no ac belt issues with 7000 limit and ltw flywheel and regularly bounced off limiter. No ac belt issues with 7200 limit and stock flywheel and regularly hit limiter. I think you have pulley mis spaced or bad pulley or bad tensioner.

[RRSperry]

Or, the belt is now damaged and the tensioner is not able to keep tension on it.

I'd replace the belt with a good part. And it's not out of the question that the new tensioner is crap too.

[ThreeD]

Going to pull the AC pulley off the crank today and do my best to insect the harmonic balancer. If it looks OK I guess I'll throw on a new AC tensioner pulley, OE BMW tensioner, and belt...

[pbonsalb]

If the balancer was bad, wouldn't you throw the main belt as well?

[ThreeD]

If the balancer was bad, wouldn't you throw the main belt as well?

Yes, although I'd imagine that if it was just beginning to go bad, since the AC pulley sits further out on the crank, it'd be more likely to be thrown off than the main belt. I looked at the balancer / damper though and it looks fine. The reason I was even suspecting the balancer was because of the multiple AC belt issues that have been going on for some time, as well as my car lunching a pressure plate strap in <10k miles, in the setting of hard driving.

Also, I noticed some "scratchy" sounds near the front of the crankcase / timing cover.

https://www.youtube.com/watch?v=iz25DbTSl9s

It's generally only noticeable from under the car at idle but the loudness of it varies seemingly randomly; sometimes you can hear it at idle from above the car as well. It gets louder with a little bit of revs (like 800-1.1k rpm), without a change in pitch/frequency, but then goes away above like 2k RPM. I'm guessing it's some sort of noise, maybe chain noise, from the oil pump...but am of course worried it's the beginning signs of a failing bearing. I figure it'll be good insurance to get a UOA prior to hitting the track in a couple weeks.

Plans going forward: UOA, new belt, and OE tensioner.

Btw, pbonsalb, RRSperry, scooper, and everyone else who has participated, THANK YOU ALL FOR THE HELP!

[M3AN]

What's the part number of the tensioner you're using?

[ThreeD]

What's the part number of the tensioner you're using?

11281717210

[M3AN]

11281717210

Interesting, that's the idiot-proof one. Is the bracket ("base plate" on the same diagram) damaged in any way? Perhaps remove that and visually inspect? Very weird.

[pbonsalb]

Isn't there a washer or spacer in there that could be installed improperly or left out? Is the tensioner using the correct shock? The shocks are different between the main belt and the ac belt tensioners. There has to be some simple explanation.

[ThreeD]

Interesting, that's the idiot-proof one. Is the bracket ("base plate" on the same diagram) damaged in any way? Perhaps remove that and visually inspect? Very weird.

Good thought. I pulled it apart again, double checked the crank bolts, and threw on all new parts: geniune tensioner, geniune belt, and INA AC pulley. Still having issues at anything >6.5k RPM. Belt falls off if I bounce it off the limiter still.

I didn't remove the base plate, but did note that in the base plate, there is a drop arm which rides on a plastic bushing connected to a dowel on the base plate. There is a little bit of play in there, maybe 2-4mm side to side, from the plastic bushing wear. That is pretty much the only bit of play throughout the entire system. I thought it was probably within normal limits, but do you think that could be causing the high-RPM belt loosening issues?

Isn't there a washer or spacer in there that could be installed improperly or left out? Is the tensioner using the correct shock? The shocks are different between the main belt and the ac belt tensioners. There has to be some simple explanation.

Great question, and I totally agree. It has been an extremely frustrating problem given how simple this system should be.

I have a thin metal mud/dirt deflector of minimal thickness behind the AC pulley, and that's it. For some reason I don't see that in the diagram, but it came on the car originally. I don't think I've tried installing the AC pulley without it, but could do that as a last resort.

What do you mean by "shock"? My tensioners have just come as one integrated unit.

Diagram linked below: http://www.realoem.com/bmw/enUS/show...diagId=11_6047

[M3AN]

...I thought it was probably within normal limits, but do you think that could be causing the high-RPM belt loosening issues?

No, because that's what the tensioner is for and it should easily take up a few mm of slack.

You probably answered this already but you've double checked the belt is the correct length? And there's no axial play in any of the pulleys?

The tensioner is specifically designed to prevent the belt flapping off. I realise it's new but have you checked how it's actually functioning? By compressing it when it's off the car? It should be pretty damn hard to compress.

[ThreeD]

No, because that's what the tensioner is for and it should easily take up a few mm of slack.

You probably answered this already but you've double checked the belt is the correct length? And there's no axial play in any of the pulleys?

The tensioner is specifically designed to prevent the belt flapping off. I realise it's new but have you checked how it's actually functioning? By compressing it when it's off the car? It should be pretty damn hard to compress.

Yep, belt is correct length. 5PK 906. The tensioner pulley itself has some axial play due to the drop arm assembly. Again minimal maybe 2mm. Rest of the pulleys are solid. I haven't pulled off or replaced the AC crank pulley so I suppose that would be next to inspect. Didn't note any problems with it though when inspecting it on the car.

I didn't try compressing the tensioner by hand but I could feel the tension when installing the belt. Feels normal, same as the other 2 tensioners I've installed recently.

[NoLastName]

I didn't remove the base plate, but did note that in the base plate, there is a drop arm which rides on a plastic bushing connected to a dowel on the base plate. There is a little bit of play in there, maybe 2-4mm side to side, from the plastic bushing wear. That is pretty much the only bit of play throughout the entire system. I thought it was probably within normal limits, but do you think that could be causing the high-RPM belt loosening issues?

I would bet the play in the drop arm is the problem. If the idler pulley isn't held firmly perpendicular to the belt and instead has some slight angle to it due to slop in the linkage, it can walk the belt off the pulleys. I had a similar issue with a worn timing belt tensioner on another car...

[pbonsalb]

There is not much else left to try.

[Braymond141]

http://www.realoem.com/bmw/enUS/show...diagId=11_6047

Are you using #10 in that diagram? Failure to use that washer will result in the pivot binding or being completely seized depending on bolt torque.

[ThreeD]

http://www.realoem.com/bmw/enUS/show...diagId=11_6047

Are you using #10 in that diagram? Failure to use that washer will result in the pivot binding or being completely seized depending on bolt torque.

yes

Brief update: installed new drop arm / tensioner bracket assembly. Revved to 7200rpm, belt still hopped its way off. Engine started making extremely loud chain noises from the head, way louder than VANOS noises or lifter tick.

Shut it off, got the car back on the ground, started it again, the engine noise persisted, but was intermittent in loudness. This is not my car, but it sounded like this: https://www.youtube.com/watch?v=b3h3...ature=youtu.be

Went for a drive to better characterize it and to see if it'd resolve itself. 1 minute in, oil pressure light came on at around 2-3k RPM, normal driving. Shut the car down, restarted, oil pressure light went away, and I started to limp it back home. Throughout the rest of the drive, the same thing happened multiple times, until I made my way home, with the noise still persisting.

So, I need to pull the oil pan, check the bearings, see if the pump sprocket is still hanging on, pull the VC, check the timing and the chain tensioners. Anything else I need to check given the intermittent loss in oil pressure?

If this ends up being an oil pump nut / shaft issue, I think its safe to call the AC pulley and oil pump issue related via crappy M5x/S5x harmonic when run at >6k RPM, meaning maybe I need a VAC/ATI damper + rebalance the clutch/flywheel assy. Any other thoughts?

TLDR: experienced likely oil pump failure

[pbonsalb]

This has been a challenge.

[ThreeD]

This has been a challenge.

I don't come up with easy cases

Say hello to my primary timing chain guide, fished out from the head:

[NoLastName]

Damn. That's bad news.

[VaTechE36]

Damn son! How does the rest of the engine look? Hopefully all of those bounce off the rev limiter tests to check the AC belt didn't kill a bunch of stuff inside the engine!

As I was catching up on the thread, I was thinking the lower timing chain tensioner might have gone it causing that rattle. The fix for the upper chain tensioner is pretty straight forward, just double check the timing of the cams when you lock everything back together with the chain.

Finally, you might want to look into the lower tensioner as well since its very easy to replace. Not exactly the cheapest, but there has been an updated version developed which really cleaned my S50 up nicely. Purrs like a kitten compared to the worn out spring style.

Good luck and sorry to hear about the tensioner problems!

[ThreeD]

Damn son! How does the rest of the engine look? Hopefully all of those bounce off the rev limiter tests to check the AC belt didn't kill a bunch of stuff inside the engine!

As I was catching up on the thread, I was thinking the lower timing chain tensioner might have gone it causing that rattle. The fix for the upper chain tensioner is pretty straight forward, just double check the timing of the cams when you lock everything back together with the chain.

Finally, you might want to look into the lower tensioner as well since its very easy to replace. Not exactly the cheapest, but there has been an updated version developed which really cleaned my S50 up nicely. Purrs like a kitten compared to the worn out spring style.

Good luck and sorry to hear about the tensioner problems!

Update to the thread: I'm actually updating it because I feel bad about my lack of forum participation, so maybe I can contribute something interesting.

Unfortunately it's slow going. My car's in MD and I'm in GA so I don't have much time to fly back and work on it. Had it pulled apart a few months ago, timing chain cover off, and...:




one of my timing chain guides was completely missing. LOL. The pieces of it are sitting in the oil pan, which I have yet to pull.

What I theorized seems to be true:

The single mass flywheel caused increased crank vibration, in combination with a raised redline (to 7200rpm), +/- failing harmonic damper / primary chain tensioner, caused the AC belt to vibrate itself off at higher RPM, and eventually caused the timing chain to break both guides. At least the oil pump, Woodruff key and crank snout look OK.

More interestingly, why does this happen? The S52 crank (and M54B30 crank) suffers from some of the worst harmonics thanks to its increased stroke compared to its M50B25 and M50B28 siblings. This causes increased harmonic damper failure, crank hub failure, which is brought on by installing a single mass flywheel. I'll post a quote from James Muskopf, the owner of RRT:

"The major disadvantage to the single mass flywheel is the lack of torsional vibration damping, which causes various oil pump chain/shaft and crank hub failures on S52 and M54B30 engines - same crankshaft. Over 7200 RPM the S52 crank really beats up the vibration damper, hub, and Woodruff key, especially bad with single mass clutch and flywheel. Improper rev match downshifting breaks pressure plate straps, and without the cushioning of the dual mass flywheel, it is much easier to fatigue and break them. Spinning the crank over 7000 for extended periods and/or many events takes its toll on the rubber vibration damper. Coupling a much heavier mass onto the back of the crankshaft with a single mass flywheel and stock diameter clutch exacerbates this problem.

"But," you say, "a single mass flywheel is lighter than a dual mass." However, understand that the dual mass flywheel decouples over half of the mass of the flywheel and the entire mass of the clutch from the torsional vibrations of the crankshaft. This reduces problems on the front of the crank, such as broken oil pump shafts and torn hub keys."

Why did this happen to my engine and not others with the same setup? Who knows.

My solution: Will pull the oil pan to take the plastic pieces out of it.
New chain, guides, primary tensioner.
ATI damper to tame the vibrations.

Going to leave the JBR flywheel for now, but next time the transmission is coming off for whatever reason, a DMF and stock clutch is going in.

[ThreeD]

Update: one year later...the issue is resolved. No more AC belt slipping off the pulley at high RPM! Finally have a drivable car with AC and a new head gasket...beyond excited.

Solution? After replacing the tensioner multiple times, compressor, belt pulley tensioner arm....The ATI damper finally fixed the AC issue.

Sent from my XT1254 using Tapatalk

[ScotcH]

Update: one year later...the issue is resolved. No more AC belt slipping off the pulley at high RPM! Finally have a drivable car with AC and a new head gasket...beyond excited.

Solution? After replacing the tensioner multiple times, compressor, belt pulley tensioner arm....The ATI damper finally fixed the AC issue.

Sent from my XT1254 using Tapatalk

Nice ... I like the ATI damper ... we spin our race engine to 7800 (when needed ... usually around 7600), and we have an 8lbs flywheel/clutch combo.

[rajicase]

Sounds like you found a band aid not really a solution. Hopefully it works out long term for you!