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

[bmwman91]

So today as I was almost home, the car started idling very poorly, bogged and died at a stop sign. It took a lot of cranking to get it to restart, and once it did, it would bog and stumble at <500RPM, and then stall. Once it is in this mode, opening the throttle makes no difference at all, other than increasing the amount of backfiring (not a lot, but I get a few good puffs).

If I give it throttle while cranking, it will start right up and run smoothly on all 4 cylinders above 1200RPM, with minimal backfiring. It was running nice and strong immediately prior to all of this, and I was out beating on it in the hills for fun. Now, if I can get it to rev above the idle speed, it will have a hesitation around 1500RPM, which was not there before.

For the last couple of months, I have been noticing that the car will bog a bit at idle for about a minute after a cold start, usually not quite enough to stall, and giving it a blip of throttle usually sorted it out. I assumed it was a dirty ICV, but cleaning it made no difference.

Stuff I do not think it is:
- Since the car runs smoothly from 1500-3000RPM (I have not had a chance to try to rev it higher, I was just happy to limp it home), I am assuming it is NOT fuel delivery related. The injectors, pump and filter all have under 30000 miles on them.
- AFM. There is no AFM because I am running a MAF conversion. If the converter was bad, it would not run well at any RPM.
- TPS. I checked the voltage output in accessory mode and it varies smoothly from 0.5V to 4V throughout its range.
- Cam position sensor. Even though the stomp test gives me code 1244, unplugging the cam sensor makes no difference. This code seems to come up for a variety of reasons other than a bad cam sensor (which is what it is supposed to indicate).
- Cracked head. No milky oil, and the expansion tank is not overflowing.
- Idle control valve. If it was this, opening the throttle would have an immediate effect. Again, if the car is in death-bog mode, nothing I do with the throttle makes a difference.
- Ignition coil. It is unlikely that more than one died simultaneously, and I know what an M42 feels like running on 3 cylinders. This ain't it lol.



My main suspicion is the crank position sensor. It is one of the few original sensors left on the engine, and I cannot think of anything else that would make the idle so bad while also causing throttle inputs to have zero effect. I measured its resistance out to be 590 Ohms, which is at the high end of the spec. God help me, the OEM ones are $232 lol (which is fine, I don't mind buying one if it the real cause!).

Any thoughts? Am I missing anything?

[monty23psk]

Long shot, the crank hub. Not sure if you got a new one with you MM engine but the rubber in between starts to crack and flex even more until breaks. Mine ran terrible a week before it failed and rubber sheared. Swapped in a used one, as new was very expensive and back on the road. The only time car left me stranded in all the years I owned it.

[bmwman91]

Thanks, I will check that too. Good idea. My damper is original.

The car started right up and idled ok this morning after sitting cold all night. I'll try warming it up and seeing if the issue reappears today.

[bmwman91]

Alrighty party people, I think that I have found an issue, and probably THE issue. It seems like it should explain my very recent no-run / poor-running problem, as well as some intermittent wonky idle and infrequent low-load backfiring over the last year. Also, it is going to be expensive lol.

Things that need replacing:
- Crankshaft damper
- Accessory drive pulley
- Crankshaft position sensor
- Camshaft position sensor
- A crank position sensor bolt with mangled head
- Cam position o-ring

I pulled the belly pan and fan+shroud to get a good look at things today. Initially it looked like it was an obviously failing crank position sensor since the cable sheath was totally broken and the shielding was exposed.




The magnetic pickup head was a little dirty, but set at the correct distance from the teeth on the position wheel.


I then took a look at the damper wheel, and thought, "well I do not believe that there is supposed to be a big circumferential gap there."


Rotating the crank with a 27mm socket & ratchet, I noted that there was a lot of axial wobble, with the damper changing distance from the timing cover by a good ~3mm throughout its rotation.




There also appeared to be some evidence that the back of the teeth were making slight contact with the cover.


At this point, a number of things under the hood were clearly in need of removal, inspection and (in a week or so) replacement. I cleaned up the crank sensor and looked a little closer. While I do not think that any of the conductors were yet failed, it is only a matter of time until oil and other contamination get into the sensor body and finish it off. Additionally, the pickup face looks partially crushed, from god-only-knows what wrenching mishap in the last 13 years.






The cam position sensor's cable sheath was also failing in a couple of spots, so it is also being replaced. I did notice the super awesome micro TIG welds holding the pickup nub to the body. I thought that those were neat.




Next I wanted to remove the pulley drive hub to get a look around behind it. This time I was fully prepared, and no sweat was produced breaking the 250+ ft-lb bolt loose. 50" cheater-bar, custom crank locking plate & a floor jack to support the plate. No problem!




The position wheel + pulley was only ~40% attached to the driving hub. I could open it up pretty easily by hand, and a little more coaxing by my foot got it looking like this.




This thing is almost exactly 29 years old. It looks like it was produced in May of 1990.


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.


Also, my engine uses a slightly different crank hub bolt than the stock E30 M42. Metric mechanic built it with this monster, and I will need to call them on Monday to get the proper torque spec for it.





You know you have been working on these things for way too long when taking the crank hub bolt off takes less than 3 minutes lol. Well, less than 3 if you don't count the trip to the hardware store for longer M8 bolts lol.

So there it is for now. I will be ordering the necessary parts to replace this old crusty crap, and hopefully the thing runs properly thereafter. Even if it is not the root cause, this stuff obviously needed replacing anyway.

[bmwman91]

Also, just for reference, here is a shot from the factory service manual that gives expected measurement values for some of the engine sensors.


My crank sensor measured 518 Ohms cold (~20C) and 590 Ohms on a hot engine. This makes perfect sense since the temperature coefficient for resistance of copper is ~0.393% per degree Celsius. That 72 Ohm increase would correspond to a ~35C temperature increase, or a sensor at ~55C, which seems reasonable. So you can expect the resistance value you measure on this, and other resistive sensors with copper coils, to vary with temperature.

[bmwman91]

Funny thing. After 20 years of working on these things, I can still learn something new (or, be dumb and not realize something). For whatever reason, the 240 ft-lb crank hub bolt gets referred to as the "crank damper bolt" a lot on forums. I have done 3 M42 timing case rebuilds over the years. At no point have I ever removed just the crank damper wheel on its own, but always as part of a timing case job. So basically, despite the fact that I have been through the motions and should know perfectly well that it is held on by 6 M8 bolts, I just went into auto-pilot mode and took off the big crank hub bolt since, "it's the damper bolt, it holds the damper on."

So, despite having the 6 M8 bolts out which freed the damper wheel, I continued to break the big bolt loose. The damper was a bit stuck on the central hub and alignment pin, so maybe if it had fallen off I would have realized that I was making more work for myself!

The crank hub seal in the lower timing case needs replacing, as I discovered. While it does not leak oil, it is sort of hard and stiff, and is not gripping the hub at all. So it is likely creating a small vacuum leak. My efforts to take the big bolt out were not totally in vain. Also, owning a tool take makes removal and installation of the big bolt a super simple affair helps to make it a couple extra minutes of work.



In other news, I took this as an opportunity to do a bunch of stuff that I bought parts for a year ago and never got around to installing. Today I replaced the thermostat and sealed up a little oil leak where the outer profile gasket has a joint with the upper timing case and inner profile gasket. The outer profile gasket and timing case gaskets were replaced as well, and the upper timing cover properly pressed flush with the rest of the valve cover sealing surfaces on the head. The engine has been running a little cool, so the thermostat needed swapping. I also put on a new viscous unit for the main fan. Tomorrow I plan to do the fuel filter and both coolant temp sensors in the head.

Hopefully the new damper wheel, sensors and lower timing cover shaft seal are stocked items so that they will arrive this week. They were ordered through getbmwparts.com.

[monty23psk]

Thanks, I will check that too. Good idea. My damper is original.

The car started right up and idled ok this morning after sitting cold all night. I'll try warming it up and seeing if the issue reappears today.

Hey man, saw your posts below with the pictures and glad you found it and looked at the damper. This happened to me and I found out because it sheared off, easy fix with a used one. Hence why I suggested it.

[bmwman91]

Here's a question...is there going to be any issue if oil has dripped down onto the clamping faces of the timing chain sprocket on the crank? The crank bolt compresses the hub, sprocket and crank together super tightly and that is how all the torque is transmitted, but since it has been apart there's probably some amount of oil that has run down and into these interfaces. I REALLY don't want to have to pull the timing case covers off since that means ordering more gaskets, and having to realign the CPS holder (since I have an M44 timing case). I don't think that I ever bothered with this in the past, and I have not heard of the lower crank sprocket slipping / shearing the woodruff key. But, if there's one thing I have learned, it is to be overly paranoid lol.

[monty23psk]

Mine was apart for about 7 days while I awaited my used hub and didn't have an issue. The engine had a top end rebuild about 5 years before that. Car still on the road and no issues.

[bmwman91]

Thanks, yeah I am just being paranoid.

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? A bunch of guys on r3vlimited have reported that the cheaper Bosch part works poorly or not at all, to the point that the car will not start.

[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.

[DesktopDave]

I just caught this - that's a crazy amount of runout on the damper! Glad you're back on the road.

Is the center section of the damper aluminum? If so, makes me wonder if the rubber doesn't have the same adhesive effect as it would with steel. Perhaps thermal expansion cycles are a contributing factor?

[bmwman91]

I didn't check before chucking it, but I would assume that the center section was aluminum since it was not rusted like the steel outer part. CTE mismatch might be a contributing factor. I also suspect 29 years, along with ~6 years on a 2.1L M42 that I drive hard, simply exceeded its design life. From the number of people that seem to be having difficulties due to this thing going out, it is probably just a lifetime thing.

[monty23psk]

Awesome news and enjoy.