Engine Electrical/Ignition Cut Out Problem SOLVED!

[TexasZinno]

Greetings,

I am having an issue with my 87' 535is and an engine cut out issue under load. You might want to sit down for this as I am at a wits end and the car had to sit out for this years TexFest sadly Any help would be much appreciated!
Problem:
When accelerating I'll feel a slight stumble, then another and if I continue to accelerate, the tach jumps and I loose all throttle response and the engine dies. Car is able to restart fine like nothing ever happened. This issue is only present on vehicle acceleration, other wise it idles perfectly and I can redline it at a stand still with no issue.

What have I done so far:

• All ignition components are new or verified good (Distributor cap and rotor, wires, spark plugs, ignition coil, ignition switch)
  Both speed and crank sensor have been replaced and tested
  Coolant temp sensor was replaced and a new connector installed.
  Thermo-temp switch tested good
  No vacuum leaks to be found. All intake boots were replaced, including the ICV hose
  Checked and bypassed the cold start valve- No result
  New fuel filter
  Fuel pressure verified good. 39/41 PSI when cutout occurs
  Voltage at the coil has been verified. 12.6V at coil when cutout occurs
  New battery as of May 2023
  Checked all common grounds that are part of the ignition circuit
  Fuse box is good: HolyGrail Labs conversion
  All relays on fuse box either replaced or tested good
  Bypassed OBC relay
  Tested both DME and AFM. Used an extra AFM that was verified to be good. Used and extra DME that was verified to be good (thanks Star).
  Replaced Timing guide rail, tensioner rail, Tensioner spring and verified proper timing in July. No notable chain/sprocket wear either.

Additional notes:
This started out as a gradual problem where it would give a slight miss here or there at highway speeds, thus I thought to do the timing service since I found the guide to be worn and on the brink of disintegration when I checked it.
Only really occurs when engine reaches operating temperature.
I did manual swap the car in 2020 and did the correct wiring/pinouts to bypass the transmission module and retrofitted the manual TPS. I have triple checked the wiring for both and tested good each time.
I replaced to rotor again since the one FCP sent me was 0.01% suspect and it ran fine for 4 days with me driving the car like a mad man. Then it started doing it again. Not sure what to think about that.

I am in the process of dissecting the wire harness to find anything. Do let me know of any suggestions that you may have or get me set up with the Ivo hotline to better assist me with this
Thanks!

[Mike W.]

You've done your homework, which is what you should do. But it's much harder for us armchair mechanics when you've already ruled out the usual suspects!

Misc ramblings.

the tach jumps

That says to me ignition, and primary, what's going to the ECU or just after it, not cap, rotor, wires etc.

This issue is only present on vehicle acceleration

Acceleration or under load? Not sure what difference it would make, but slowly winding out to 6K RPM or sitting in the driveway? Will it slowly, but smoothly, pull to 5500-6K. The tach issue would be ignition, but a lean mixture can do funny things too.

Extremely long shot. I had an E24 long ago that would cut out over about half throttle, L jet car, which is very similar to Motronic, except for ignition control. Pre internet, wasn't smart enough in those days to look at the tach. But I could gradually run it up to redline and be fine. But if I floored it, bam, it would cutout. Turned out to be a broken wire going to the CTS plug that would make contact, except when the engine torqued over and pulled on said wire. Unlikely to be that specifically, but keep that sort of thing in mind. Torquing, stretching wires, not a component failure.

[Kerrvillian]

Turned out to be a broken wire going to the CTS plug that would make contact, except when the engine torqued over and pulled on said wire. Unlikely to be that specifically, but keep that sort of thing in mind. Torquing, stretching wires, not a component failure.

I've been helping (or at least trying to help) Al with this problem since it started. The initial issue was a very irritating ignition miss at low RPM's when accelerating in a specific RPM band. The timing chain/guides were a suspect since we both helped another forum member (HitmanX) with his car earlier in the year with broken guides. Al's guides were still intact, but on their way out, but his tensioner spring was kaput.

He's heard the lecture about parts-cannon approaches to fixing cars, so let's spare that, but we have gone through and methodically verified several key suspects and ruled them out.

Important things to note/add to this conversation:

- Grounds have been verified good, cleaned, recrimped, etc.
- Crank and reference sensors were aftermarket and tested at 1.4KΩ, way out of spec. Replaced with OEM sensors from BMW, no change.
- Test drives with multimeters attached prove the fuel pump and ignition coil never lose power
- Engine never stalls or causes problems when the car is stationary
- Al's girlfriend states that cutouts when driving always follow a bump in the road
- DME ruled out with known-good spare

Additional weirdness: The car seems to run poorly with the oxygen sensor in the circuit. Idle is rough and the car seems to run rich for some reason when hot. O2 sensor has been replaced and the intake system smoke-tested. We're reasonably certain there's no vacuum leak or AFM problem with this car. The O2 sensor weirdness is present with the stock DME, a spare DME, and with a Mark D'Sylva chip installed.

Everything screams to me that there's a loose connection somewhere or a fractured wire that's making/breaking based on jostling of the chassis or torquing of the engine. Finding which one it is? Anyone got lucky lotto numbers? Rubber chicken to shake at the car?

[adam_poll]

So I've had cuts like this in the past in a few different ways but you have ruled most of those out. I've had the DME do it, reflowing the solder solved this (or a spare), not your problem. I've had a shorting high voltage cable from the coil to the distributer cause this, not your issue either as you've replaced that stuff.

With the latest comments about the oxygen sensor though I think it is worth looking at that a bit more. I think with the 87 BMW moved to the long 02 sensor wires and the connector on the firewall rather than having it under the car like on the 86 and earlier. The reason they moved it was water could get into the connector down there causing the car to cutout. I've had this happen to me in the past with my 86 in heavy weather. All this to say issues here can cause symptoms like you are experiencing. I think it is worth spending some more time here checking the harness on the car side going to the connector, the heater relay etc. My car always ran nicer without the 02 sensor (steadier idle etc.) but had a loss of fuel economy.

The replacement rotor is interesting, I don't know what to make of that one but I'd be inclined to look a bit deeper into that. Is the centre post contacting well, is the depth of the rotor right (I don't know what it should be or how it would move)?

Lastly and maybe this has been tried but at idle (or revved) go through the car under the hood (and even the ECU connector in the glovebox) pushing, pulling and prodding connectors and lengths of the harness (there are connections buried in there) trying to get the car to stumble or die. If you can replicate it you should have your smoking gun.

[Mike W.]

Between

the tach jumps

and

Al's girlfriend states that cutouts when driving always follow a bump in the road

It sure sounds like a wiring problem, not a component problem. Although one thing just came to mind, the OBC and relay box. Seems like more and more of them are failing and while I don't recall this specific problem, that doesn't mean it's not. Might try by passing it at the very least just for testing.

If that's not it, try pulling and pushing on all engine wiring, especially ignition. Knock on things, jiggle them, abuse them, with it running even just at idle.

Regarding running rich, that sounds like an additional, probably unrelated problem. But M30s of this era have never had a particularly smooth idle and not being there I obviously can't say if it's the usual M30 idle or different. Typically they do idle smoother with the O2 disconnected as by design they're always going back and forth between a little rich and a little lean. Check the O2 sensor output to get a better idea on rich/lean.

and the engine dies. Car is able to restart fine like nothing ever happened

That kind of suggests the ECU is either losing power or the speed sensor signal.

[TexasZinno]

So I've had cuts like this in the past in a few different ways but you have ruled most of those out. I've had the DME do it, reflowing the solder solved this (or a spare), not your problem. I've had a shorting high voltage cable from the coil to the distributer cause this, not your issue either as you've replaced that stuff.

With the latest comments about the oxygen sensor though I think it is worth looking at that a bit more. I think with the 87 BMW moved to the long 02 sensor wires and the connector on the firewall rather than having it under the car like on the 86 and earlier. The reason they moved it was water could get into the connector down there causing the car to cutout. I've had this happen to me in the past with my 86 in heavy weather. All this to say issues here can cause symptoms like you are experiencing. I think it is worth spending some more time here checking the harness on the car side going to the connector, the heater relay etc. My car always ran nicer without the 02 sensor (steadier idle etc.) but had a loss of fuel economy.

The replacement rotor is interesting, I don't know what to make of that one but I'd be inclined to look a bit deeper into that. Is the centre post contacting well, is the depth of the rotor right (I don't know what it should be or how it would move)?

Lastly and maybe this has been tried but at idle (or revved) go through the car under the hood (and even the ECU connector in the glovebox) pushing, pulling and prodding connectors and lengths of the harness (there are connections buried in there) trying to get the car to stumble or die. If you can replicate it you should have your smoking gun.

I drove with the kick panel off and jostled as many wires as I could when driving including the connector to the DME. Also good to mention that the car cuts out regardless if the O2 sensor is plugged in or not. But its good to know about the change of the sensor for later cars. Thanks

[TexasZinno]

Al's girlfriend states that cutouts when driving always follow a bump in the road

It sure sounds like a wiring problem, not a component problem. Although one thing just came to mind, the OBC and relay box. Seems like more and more of them are failing and while I don't recall this specific problem, that doesn't mean it's not. Might try by passing it at the very least just for testing.

I did bypass the relay box to no result. To clarify, yes this is just about guaranteed a wiring issue but I've driven the same bumpy road multiple times to test and only found one time it acted up, which makes narrowing this down a lot more difficult.

[Profile36]

I have seen similar problems on a couple <95 cars that I own. The e36 would act up when the o2 sensor was bad. On the e28, it was the in tank fuel pump which was no longer functional and rust in the fuel tank was clogging things up.

I know you verified your fuel pressure and did not notice any dips, but are you certain the pump is functional and the tank is clean? It only takes a few minutes to verify and might be worth a try.

Another simple one, have you verified the plugs are gapped correctly? Do they show any sings of predetonation or fouling?

For the speed sensor, you replaced the one on the transmission, not the one in the differential, correct?

As far as changes you have made in the past. What were some of the significant ones which are recent to the problem starting? Could it be related to the fuse box mod? Is that close in terms of timeline to when the problem started occurring?

-M

[TexasZinno]

I have seen similar problems on a couple <95 cars that I own. The e36 would act up when the o2 sensor was bad. On the e28, it was the in tank fuel pump which was no longer functional and rust in the fuel tank was clogging things up.

I know you verified your fuel pressure and did not notice any dips, but are you certain the pump is functional and the tank is clean? It only takes a few minutes to verify and might be worth a try.

Another simple one, have you verified the plugs are gapped correctly? Do they show any sings of predetonation or fouling?

For the speed sensor, you replaced the one on the transmission, not the one in the differential, correct?

As far as changes you have made in the past. What were some of the significant ones which are recent to the problem starting? Could it be related to the fuse box mod? Is that close in terms of timeline to when the problem started occurring?

-M

No dips in fuel pressure present. Fuel pressure was constant at 39/41 PSI. New FPR and fuel pumps too. I have a set of champion non resistor plugs. they were slightly on the rich side but otherwise good. I went back to the Bosch WRLS plugs just to be sure and no change. Both crank and speed sensors in the bellhousing were replaced and verified resistance per spec. I checked the fuse box again and all related grounds and didn't find any discrepancies.

I forgot to mention that this issue seemingly only occurs once the engine it at operating temperature i.e. once it exits the warm up loop. I started to take apart the wire harness coming in and out of the fuse box and didn't find a smoking gun.

[Profile36]

I have seen similar problems on a couple <95 cars that I own. The e36 would act up when the o2 sensor was bad. On the e28, it was the in tank fuel pump which was no longer functional and rust in the fuel tank was clogging things up.

I know you verified your fuel pressure and did not notice any dips, but are you certain the pump is functional and the tank is clean? It only takes a few minutes to verify and might be worth a try.

Another simple one, have you verified the plugs are gapped correctly? Do they show any sings of predetonation or fouling?

For the speed sensor, you replaced the one on the transmission, not the one in the differential, correct?

As far as changes you have made in the past. What were some of the significant ones which are recent to the problem starting? Could it be related to the fuse box mod? Is that close in terms of timeline to when the problem started occurring?

-M

No dips in fuel pressure present. Fuel pressure was constant at 39/41 PSI. New FPR and fuel pumps too. I have a set of champion non resistor plugs. they were slightly on the rich side but otherwise good. I went back to the Bosch WRLS plugs just to be sure and no change. Both crank and speed sensors in the bellhousing were replaced and verified resistance per spec. I checked the fuse box again and all related grounds and didn't find any discrepancies.

I forgot to mention that this issue seemingly only occurs once the engine it at operating temperature i.e. once it exits the warm up loop. I started to take apart the wire harness coming in and out of the fuse box and didn't find a smoking gun.

While not completely omitting the fact that this could be a wiring issue, I think the chances are, it’s still something else, it sounds like it’s fuel mixture related, one of the components might be causing misreading, and the fuel enrichments might be off causing issues while the engine is hot. I suggest you don’t give up and continue testing.

1. How did you test the intake system for unmetered air? Smoke tests are the best in my opinion. If you have a small pancake compressor at home, you can buy a cheapo paint can smoker for next to nothing, mine has proven to be invaluable.
2. You replaced the o2, but did you try to verify there is voltage drop when the car is running/warm and unmetered air is introduced? I think this test is very important. Is the o2 heater relay in good standing?
3. You replaced all the components responsible for fuel enrichments, except the throttle switch, have you tested and made sure it’s good, wiring ok since the manual swap?
2. Are we certain the injectors are in good standing? Have you checked that the pressure does not bleed off when the car sits? Should be easy to leave a gauge on it overnight. Chances are that’s not it, since it does the opposite, runs well when cold not hot, not typical for clogged injectors but worth ruling out.
3. Have you tested and tried blowing into the emissions purge valve to see that it works when it’s supposed to? It could be sucking in fuel from the canister at WOT.

Ideally, if you could put a wideband on it, I think it could greatly help with resolving this, otherwise it’s more of a guessing game. Hope you find it soon!

-M

[TexasZinno]

No dips in fuel pressure present. Fuel pressure was constant at 39/41 PSI. New FPR and fuel pumps too. I have a set of champion non resistor plugs. they were slightly on the rich side but otherwise good. I went back to the Bosch WRLS plugs just to be sure and no change. Both crank and speed sensors in the bellhousing were replaced and verified resistance per spec. I checked the fuse box again and all related grounds and didn't find any discrepancies.

I forgot to mention that this issue seemingly only occurs once the engine it at operating temperature i.e. once it exits the warm up loop. I started to take apart the wire harness coming in and out of the fuse box and didn't find a smoking gun.

While not completely omitting the fact that this could be a wiring issue, I think the chances are, it’s still something else, it sounds like it’s fuel mixture related, one of the components might be causing misreading, and the fuel enrichments might be off causing issues while the engine is hot. I suggest you don’t give up and continue testing.

1. How did you test the intake system for unmetered air? Smoke tests are the best in my opinion. If you have a small pancake compressor at home, you can buy a cheapo paint can smoker for next to nothing, mine has proven to be invaluable.
2. You replaced the o2, but did you try to verify there is voltage drop when the car is running/warm and unmetered air is introduced? I think this test is very important. Is the o2 heater relay in good standing?
3. You replaced all the components responsible for fuel enrichments, except the throttle switch, have you tested and made sure it’s good, wiring ok since the manual swap?
2. Are we certain the injectors are in good standing? Have you checked that the pressure does not bleed off when the car sits? Should be easy to leave a gauge on it overnight. Chances are that’s not it, since it does the opposite, runs well when cold not hot, not typical for clogged injectors but worth ruling out.
3. Have you tested and tried blowing into the emissions purge valve to see that it works when it’s supposed to? It could be sucking in fuel from the canister at WOT.

Ideally, if you could put a wideband on it, I think it could greatly help with resolving this, otherwise it’s more of a guessing game. Hope you find it soon!

-M

1. I did actually smoke test the intake and its all sealed.
2. Definitely will check the heater relay for the o2 and confirm voltage. Bothersome factor is that this issue occurs whether or not the o2 is plugged in.
3. I have tested the throttle switch and verified it works as it should and even used an old bosch one from a parts car to be extra sure.
4. I replaced the injectors with reman ford mustang injectors and replaced the o rings about a year ago. The connectors were also replaced with the EV1 style of connector and verified that all injectors and wiring are working properly.
5. The evap system was bypassed around the same time as the injectors were replaced (used a vacuum cap for the hose going to the intake boot. As a safe measure I hooked it back up to no result.

I'm almost done with this harness, if I don't find anything I'll proceed with the suggestions. Thank you again.

[TexasZinno]

Its been a hot minute but after cleaning up the harness and ohm-ing out every wire and sensor I can conclude that it wasn't the harness or a wiring issue. The culprit in the end was the alternator of all things. If you recall, the alternator generated the proper 13.1V and had a new(ish) voltage regulator in it. Only after hooking up to a tester and oscilloscope, we'd find that a single bad diode in it caused an inconsistent sine wave that more than likely bugged out the ignition system. Put in a new alternator and all symptoms went away! It just made the round trip from RadWood Austin 2024 without a hiccup. Thank you all for your input and suggestions

[Mike W.]

Only after hooking up to a tester and oscilloscope, we'd find that a single bad diode in it caused an inconsistent sine wave that more than likely bugged out the ignition system.

Wow, glad you got it figured out. But if that's not a facepalm I dunno what is, I mean that's really out there. 13.1 charging voltage is low, but I wouldn't have expected a problem like that. Nice troubleshooting.

[gadget73]

Interesting, but it makes sense. Definitely not a normal or expected fault and not one that would be easy to hunt down without a scope or just throwing parts at it in desperation.