M70 Dropping into Limp home mode when warm.

[dragon850]

I posted this on E32 forum because it's for my 750il but would like to collect some wisdom here as well :

Hey guys, I am working on restoring my 1991 750il and found myself quite puzzled.
Car would start on cold most of the time on 12cyl and run perfect till driven around for about 5 to 10 min when it suddenly drops the drivers side bank.
Things done so far without any change:
1) New Bremi spark plug wires, caps and rotors, proper BRK6EK plugs.
2) New Coil
3) Another known good working Maf
4) Brand new fuel pumps and rebuilt fuel pump assembly. (fuel pressure 3bar)
5) Ohmed out the Crank Sensors both read around 550 ohms between pins 1 and 2 and a little higher around 590 when hot.

Diagnostic is showing Error 16 for cylinder position sensor, that got me going down the rabbit hole of thinking maybe new Bremi sensor is defective, so I again pulled the new wires, ohmed every wire and installed another known working CPS.
No change, that was frustrating because you all know how hard it is to change the wires on drivers side bank.
I am getting perfect 3 bar fuel pressure on the dead bank.
No other errors in the system.
Is it possible for crank position sensors still be bad even if they ohm out ok?
Any other ideas?

[LappingLuke]

Is it possible for crank position sensors still be bad even if they ohm out ok?
Any other ideas?

I obviously haven't had my car long enough to know specifically, but in my experience, ANYTHING can be dead on arrival especially ​small electronic components...

[8eights]

I've seen this from FPR"s and IMG's

[The Pondman]

As you know I'm new to the 850 but if your symptoms were happening on one of my old street cars I would suggest checking temperature and pressure related sensors and switches.

[shogun]

CPS real good testing is only possible with osci to see the waves. from workshop manual
distributor rotor 1+/-20% kohm, angled/shielded connectors 1+/- 20% kohm, spark plug connectors 5+/- 20% kohm, cylinder identification sender coil resistance at 20 degree C (68F) <1 ohm, pulse sender/CPS coil resistance 540 +/- 10% ohm

Make a test and swap the 2 DME's and see if the fault moves to the other side, if so, DME could be the problem. We did it like this https://1.bp.blogspot.com/-BXlVsKrAv...0/IMG_4275.jpg
2 DME and EML swap test https://1.bp.blogspot.com/-1Yuj0BB-6...0/IMG_4276.jpg

[dragon850]

CPS real good testing is only possible with osci to see the waves. from workshop manual
distributor rotor 1+/-20% kohm, angled/shielded connectors 1+/- 20% kohm, spark plug connectors 5+/- 20% kohm, cylinder identification sender coil resistance at 20 degree C (68F) <1 ohm, pulse sender/CPS coil resistance 540 +/- 10% ohm

Make a test and swap the 2 DME's and see if the fault moves to the other side, if so, DME could be the problem. We did it like this https://1.bp.blogspot.com/-BXlVsKrAv...0/IMG_4275.jpg
2 DME and EML swap test https://1.bp.blogspot.com/-1Yuj0BB-6...0/IMG_4276.jpg

I have installed another good known DME with no change.
Are you talking about Crank Position Sensor testing? I have swapped a new BMW cylinder ID sensor with another one, that pretty much eliminates that.
Have you ever seen a bad Crank sensor even if it ohms out good?

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I've seen this from FPR"s and IMG's

Getting proper 3 bars on the dead bank. Injectors not firing.
What's an IMG? Sorry

[LappingLuke]

Can you associate the start of limp mode with a certain coolant temp perhaps?

[8eights]

FPR---Fuel pressure regulators

IMG---Intake manifold gaskets

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[dragon850]

Can you associate the start of limp mode with a certain coolant temp perhaps?

It's pretty random but it does drop when the car is warm and driven a bit...
The coolant temp sensors provide info to both ecus so don't think it's that.

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FPR---Fuel pressure regulators

IMG---Intake manifold gaskets

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I smoke tested the engine, no leaks....

I think I will move on to working on replacing both crank position sensors... I have 2 new BMW oem sensors laying on the shelve.

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I will also continuity check the harness to CPS and Cyl position sensor on the drivers bank. Then deoxit clean all the connectors.

[CD05001CIA]

It's pretty random but it does drop when the car is warm and driven a bit...
The coolant temp sensors provide info to both ecus so don't think it's that.

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I will also continuity check the harness to CPS and Cyl position sensor on the drivers bank. Then deoxit clean all the connectors.

I had a similar situation I thought was heat but really was just from driving the car. The vibration caused fractured wires within a harness to crosstalk. Just thought I'd throw that out there as a remote possibility.

[ChrisFuture8]

Hey Ed. LHM is the story of my life. Last time I had that on my car was while with the original motor still in the car. Obviously, it wasn't that motor that went into limp home mode but the electronics that caused it.
One of the CPS that I thought was good was the cause. It went slowly. Car started running rough right when I exited the freeway and waited at the stop light. Upon acceleration, it sputters and protested. Black smoke was coming from the tail pipe. I traced the problem to the CPS eventually and proceed with spraring some cleaners between the sensor and the geared wheel. That was a bad thing to do as it killed the CPS completely.

If your 750il runs bad with improper mixture and black smoke at the exhaust tip, it might be the CPS...depending on which exhaust the black unburned fuel is coming out of.

Other possible cause is the engine wire harness. Little cracks in the aging wires will cause intermittent continuity issues once the temp under the hood is hot enough where the plastic sheathing on the wires expand and allow the leads to separate ever so slightly. I also had that happen to me on my original engine wire harnes. It happens when I'm on the freeway slowing down or taking the exit ramp and try to acclerate into traffic again.

Old wire harness can cause all sorts of intermittent connectivity issues.

[rjjablo]

I had one of the 4 Relays in the EML Box get flaky. Hit a bump and into Limp mode. $20 later and that went away

[dragon850]

Hey Ed. LHM is the story of my life. Last time I had that on my car was while with the original motor still in the car. Obviously, it wasn't that motor that went into limp home mode but the electronics that caused it.
One of the CPS that I thought was good was the cause. It went slowly. Car started running rough right when I exited the freeway and waited at the stop light. Upon acceleration, it sputters and protested. Black smoke was coming from the tail pipe. I traced the problem to the CPS eventually and proceed with spraring some cleaners between the sensor and the geared wheel. That was a bad thing to do as it killed the CPS completely.

If your 750il runs bad with improper mixture and black smoke at the exhaust tip, it might be the CPS...depending on which exhaust the black unburned fuel is coming out of.

Other possible cause is the engine wire harness. Little cracks in the aging wires will cause intermittent continuity issues once the temp under the hood is hot enough where the plastic sheathing on the wires expand and allow the leads to separate ever so slightly. I also had that happen to me on my original engine wire harnes. It happens when I'm on the freeway slowing down or taking the exit ramp and try to acclerate into traffic again.

Old wire harness can cause all sorts of intermittent connectivity issues.

Thanks Chris, well I have an update as of last night and this morning.
With Cartoonz help I have checked the continuity of pins 16/44 (cylinder position sensor) and 67/67 Crank position sensor while the car was in limp home mode. All pins have continuity even when I wiggle connectors and wires around.
This morning i swallowed a frog and spent 2 hours replacing the drivers side crank position sensor with a brand new BMW one.
Started on all 12 drove it for about 10min perfectly but when the car fully warmed up I came to the stop light and boom bank dropped. Could not restart it on 12 again
Scratching my head and other parts of my body now ... this is weird.

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I had one of the 4 Relays in the EML Box get flaky. Hit a bump and into Limp mode. $20 later and that went away

Replaced both ECU relays in there already.
Fuel pumps have full power.

[CD05001CIA]

I had a similar situation I thought was heat but really was just from driving the car. The vibration caused fractured wires within a harness to crosstalk. Just thought I'd throw that out there as a remote possibility.

Let me be more specific... the insulation was fractured on the individual wires causing the wire's conductors within the harness to crosstalk.

[ChrisFuture8]

This morning i swallowed a frog and spent 2 hours replacing the drivers side crank position sensor with a brand new BMW one.
Started on all 12 drove it for about 10min perfectly but when the car fully warmed up I came to the stop light and boom bank dropped. Could not restart it on 12 again

It was the same bank that dropped into LHM, Ed? Since you replaced the crank position sensor and the problem is still there, it sounds like a signal problem still. The problem may be upstream...possibly somewhere in the engine wire harness. It may be the wires to the plug for the crank position sensor. Again, when the temp under the hood gets warm enough, the silicon wire jackets loosen up and the wires inside can relax.
Nex time the car goes into limp home mode, you may want to check continuity from the plug back to the DME connection. Likely a higher OHM reading is an indication the wire(s) is cracked and since the silicon jacket outside is softer it allows the wires inside to flex/bend.

Also, sometimes, I hear very audio clicking sound coming somewhere between the ignition coil, the ignition coil lead, and the rotor when the underhood temp is hot...especially if the car is at the shop idling and waiting for a smog check run. This also cause the car to go into LHM. Unfortunately, these intermittent continuity problems is a bitch to chase especially if you're doing it yourself...

[shogun]

here is a pic of an E32 750 with similar problem which drove erwin crazy, but he found it : Weird electrical problem - throttle wiring is hinky, sends DK into spasm http://www.bimmerboard.com/forums/posts/644209/
we also have found on some E32 750 broken wires which sometimes had contact/sometimes not in the wire loom to the MAF, just before the wire loom plugs into the MAF, the outer sheath has become super hard and when it breaks it cuts into the single wires in the loom. I replaced that hard stuff complete with new electrical insulating tape, that type BMW uses for harnessing https://www.angryasssolutions.com/parts/harnessing-tape

[petejk]

I replaced that hard stuff complete with new electrical insulating tape, that type BMW uses for harnessing https://www.angryasssolutions.com/parts/harnessing-tape

That looks like a stronger version of Tesa tape - which I’ve been using on all of my wiring repairs so far.


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[dragon850]

Guys, I figured it out!
Thank you to everyone that chimed in!
The problem was absolutely stupid and simple, previous owner installed the correct spark plugs but did not tighten the #12 all the way!! Probably because the insanely tight space.
That was causing the cylinder position error!
This just proves you should never give up on digging and you will always eventually find the problem!
She is fixed and runs so smooth!
Goes like a bat out of hell, probably can give my sc 850 a run for its money!
One of 12 produced, and one of the last ones Dinan made in 1997.


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[8eights]

Glad to hear it, You were on the correct path though...

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[LappingLuke]

Damn! Glad it was simple you were dancing around the solution all long!

[sshivaram]

Super !

[ChrisFuture8]

Nice! Great find too!

[dragon850]

Sooooo..... I have an update.
After posting victoriously that the car is fixed and drove perfectly for a day I was stoked, that turned to agony next day when it started on 6 again!!
Didn't want to post anything till I figured it out, after going through every system again and my frustration meter maxing out finally I found a sneaky bastard!!
It was not part of the regular car systems at all, it was damn Dinan Knock control box!! Major thanks to cartoons for this one!

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[cartoonz]

"You" figured it out?
That's a bit of a stretch.... lol

[paulmer]

Wait, I thought only M73 cars had knock sensors? As part of the TT setup, did Dinan retrofit knock sensors? How would they tie in to the factory DMEs?