M42 Won't Idle, Some Backfiring [EDIT: Photo Dump Of Broken Stuff]

[jrobie79]

Seems like it would be better to look into aftermarket or custom made dampers for that price.

Possibly. This is such a critical component, both for engine reliability/longevity and fundamental ECU operation that it is definitely not worth using anything that is not of the same quality as OEM though. A custom one that was properly engineered would probably cost $500-1000.

I had a nightmare of a time diagnosing my no start issue, to the tune of 2+ months (it's not my daily so it wasn't extremely critical), but it ended up being the damper. There's a thread floating around here from 2017 describing my process, it sucked.

[wazzu70]

Who knows, maybe they adjusted the elastomer section for whatever small difference in rotational inertia there is with the serpentine pulley, but then it would still be optimal for an M42 with a serpentine setup! It is $200 less than the V-belt version, anyway.

Seems like a good reason to update to serpentine setup!

[bmwman91]

Who knows, maybe they adjusted the elastomer section for whatever small difference in rotational inertia there is with the serpentine pulley, but then it would still be optimal for an M42 with a serpentine setup! It is $200 less than the V-belt version, anyway.

[wazzu70]

I have no idea if the later serpentine damper is "better" and I would guess that it is simply a correct part for its intended design application. Some people have converted to the serpentine belt setup.

Converting to the E36 M1.7.2 ECU is easier in terms of wiring and hardware, but still requires some serious software modifications that I do not believe anyone has ever finished or run an E30 M42 on. M1.7.2 is basically the same thing as M1.7 in many ways, with (supposedly) some improvements to the idle control algorithms. It is not likely to be worth the effort to convert to that.


Take a look at the photos I posted earlier in the thread. Those sort of show the adapter bracket. I am not going to remove it since I do not want to mess up the alignment it has to the damper wheel. It is not something that I would really try to make myself since it would be much faster and probably cheaper to buy a pre-made one which is known to be correct. I am not sure how much Metric Mechanic would want for one, but I can't imagine that it would be too terribly expensive.

I doubt the damper for the serpentine setup is any different than the V-belt internally. Maybe some provision difference for the hub, but not to the elastomer itself. The change in belt/pulley would not impact the TV of the engine...so why would they bother designing a new damper?

The OBDII M52 engines have the tooth gap in the same place as their OBDI counterparts, so I assume M44 and M42 are the same. Assume is the key word, I don’t know for sure.

[bmwman91]

Possibly. This is such a critical component, both for engine reliability/longevity and fundamental ECU operation that it is definitely not worth using anything that is not of the same quality as OEM though. A custom one that was properly engineered would probably cost $500-1000.

[jwal]

Seems like it would be better to look into aftermarket or custom made dampers for that price.

[bmwman91]

Man, you got a deal! Yeah these suckers are expensive now. Since the production date on my new OEM one, shipped straight from a BMW warehouse in Germany, was 10 years ago, I suspect that these are going to be NLA very soon.

[jrobie79]

A little more light pushing by hand completed the transformation into scrap.


I also noticed that the accessory drive pulley was a little bent up. Since the new damper wheel is ~$500, I am not going to bolt a bent thing to it (nor am I going to bother straightening it). It is getting replaced since it is simply not worth throwing the balance out or otherwise distorting the new damper drive hub.

I cannot believe how expensive these things are. About 18 months ago I had the same problem, car died one day, would never start back up. I diagnosed nearly every wire from the ECU to the main fuse panel, etc... Then my dad suggested I look at the crank vibration damper, and sure enough the toothed wheel was separated from the rubber.

I jumped on to BavAuto (RIP ) and found OE BMW was listed at $99 - I should have bought 10 of them looking back. Someone else posted in this thread similar symptoms and I told them to check the VD and look at Bav's website for the part, it was only $99....when I went back their website I saw it jumped over $500, and this was only about 6 months after I purchased mine. Got lucky I guess.

[bmwman91]

I have no idea if the later serpentine damper is "better" and I would guess that it is simply a correct part for its intended design application. Some people have converted to the serpentine belt setup.

Converting to the E36 M1.7.2 ECU is easier in terms of wiring and hardware, but still requires some serious software modifications that I do not believe anyone has ever finished or run an E30 M42 on. M1.7.2 is basically the same thing as M1.7 in many ways, with (supposedly) some improvements to the idle control algorithms. It is not likely to be worth the effort to convert to that.


Take a look at the photos I posted earlier in the thread. Those sort of show the adapter bracket. I am not going to remove it since I do not want to mess up the alignment it has to the damper wheel. It is not something that I would really try to make myself since it would be much faster and probably cheaper to buy a pre-made one which is known to be correct. I am not sure how much Metric Mechanic would want for one, but I can't imagine that it would be too terribly expensive.

[jwal]

Would you mind taking a picture of what they made? I can design something myself, but it would be nice to have somewhere to start. Is the late M42 multi-v belt damper better? Using OBD I would be an easier conversion than OBD II, but would still see improvements in hardware and software I would think.

[bmwman91]

Yeah, the M44 case has an 11mm thick pump rotor (vs 10mm for the M42 IIRC).

There is no provision for the crank sensor, and you need a custom machined mounting bracket. Metric mechanic made one for my engine and they would probably sell you one.

I also looked into a conversion to Motronic 5.2, which the M44 uses. Hardware-wise it is doable and requires some fairly in-depth modifications to the wiring harness. The thing I do not know is how the timing on the M44 crank wheel compares to the M42 (if the gap is at the same crank position). There is also a lot of work that needs to be done in the firmware, though, and I do not think that there is anyone out there who knows M5.2 well enough to do it since it was not a popular ECU with tuners as far as I know. Honestly, this thing runs so well with the new damper wheel and sensor that I really think that many people's idle and WOT issues are coming from 25-30 year old parts. The toothed damper wheel and its sensor are THE single most critical parts for the ECU to do its job properly.

[jwal]

since I have an M44 timing case

I didn't think you could use an M44 timing case and keep the front mounted crank sensor from the M42. Would you mind sharing how you did that?

I am waiting on my late M42 block to get back from the machine shop so I can put my motor together. I've been wanting to go OBD 2, using all the M44 stuff, but I would like to be able to install and run everything without messing with the ECU and all that first. Only issue with doing that is using a different timing case. I have a late M42 case, and an M44 case, but everything I've read states the M44 is superior, and the oil pump has more flow, right?

[wazzu70]

Nice, at least it solved the issue!

[bmwman91]

Well, whatever the case may be with the "genuine" Bosch crank sensor, it works. I got it all back together today, and the car runs better than ever before. No more wonky idle when coming off the throttle, and it is dead smooth all the way up to 7500RPM (versus a sort of lumpy seeming upper powerband before). I suspect that the damper has been on its way out for years, maybe the sensor too, and with fresh new stuff in there the ECU can actually do its job again. This was well worth the considerable expense to replace.

[bmwman91]

I received my parts today. The crank position sensor that I paid $232 for is a Bosch part. There seem to be two versions of it, or at least two photos of it that sellers have. The places selling the $232 part show a Bosch one with an original looking plug such as this:

https://www.ecstuning.com/b-genuine-bmw-parts/crankshaft-position-sensor/12141721504/

The places selling the $80 one all have this as the photo:

https://www.autohausaz.com/pn/BS-12141721504

So, what is the deal? The original cam & crank sensors have the BMW logo on the sensor body and a "DH" logo, or what looks like those letters. The new cam sensor is this way, but the crank sensor is Bosch. Can anyone who recently bought a "Genuine BMW" sensor let me know what it looks like, because if I got a crap sensor I am just going straight for a proper replacement or a refund, although at this point I cannot seem to find anywhere that sells one that looks like a non-Bosch one. Does Bosch make 2 versions, with the cheaper one being crap?