'88 M6 Hot Start Issues

[Griphish]

Hello everyone,

I've been having issues with my 1988 M6 as of late and after a lot of searching on here and trying different tests I'm still stumped. Hopefully one of you guys can point me down the right direction from here. I'm having issues when trying to start the car after it is hot. It fires right up and runs decently well when everything is cold but if I drive around it will eventually stall and not want to start. I can recreate this issue by letting the car idle in the garage and then trying to start it after its hot.

I've been working to get this M6 back to good working order over the last year and a half. When I brought it home the car had not been on the road for about 8 years so I have replaced the following parts to get the car running.

• Air Filter

• All rubber vacuum and fuel lines in the engine bay

• Reset throttle bodies to factory spec and reset TPS

• Replaced spark plugs, distributer cap, rotor, wires, and coil

• Replaced all 6 injectors, both fuel pumps, and fuel pressure regulator

• New oil fill cap

• New fuel pump relay

• Replaced both speed and reference sensors

• New main relay

• New coolant temperature sensor (ECU)

I've gone through some trouble shooting steps but haven't been able to solve the issue.

• Cleaned ICV, no change

• Removed ECU and re soldered the 3 transistor solders. This did help smooth out the idle but did not help the no start issue.

• Tested fuel pressure, Getting 2.8 BAR at idle. was getting that on the old regulator and the new one. tested vapor lock and the pressure holds for at least an hour after turning the engine off. Also tried jumping the fuel pump for a couple minutes before trying to start and no change.

• Checked for spark while hot; no spark. Checked coil resistance. Terminal 1 to 15 resistance is 0.7. 15 to center is 6870 when cold and 7180 when hot. both terminal 1 and 15 have voltage when key is in the ON position.

• Checked main relay, have voltage at pin 86 and 30. have voltage at 87 and 87b when key is in ON position.

• Speed sensor voltage on start is 0.024V, Ref. sensor is 2.02V

I'm have the sneaking suspicion that the DME may be pooched. If that's the case I'm contemplating going all in and getting an ECUMaster EMU Classic and modernizing the engine a little bit.

Any help on this would be much appreciated!

Cheers!


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[RSheiman]

Make sure all the ground connections are clean. Did you do all those things listed in trying to solve this or did you do some and the problem arose?

[Griphish]

I've tried all of these steps listed and still no dice.

I've also tried idling the car until I get it hot and have the no start issue. Once it is hot I took the DME out and put it in the freezer for 10 minutes. When I plugged it back in it was still not starting.
Not sure if that rules out the DME or not.

[RSheiman]

Swap out the main relay with the new fuel pump relay or just change the main relay. Also forget this if both are new. In your list, you have fuel pump relay twice so perhaps one was fuel, other was main.

Prob not the DME. You can send it to Programa if need be to test it. Was cheap a while back. They can fix it if necc. Certainly cheaper than new and you will not find a used one.

[Griphish]

Oof! That was a typo. You're right Rob, one was fuel relay and the other was main relay. I've got it corrected now.

I've just ordered an oscilloscope so I'm going to see if I can test the CPS and Speed Sensor with it. I'm also going to see if I can test the ignition switch too. I believe I can jump the battery to the coil and if it still doesn't start then it's not the switch. Let me know if that logic makes sense though.

[RSheiman]

When the car dies, does it slowly die out or is it almost an instantaneous thing?

[Griphish]

It only stalled out one time when I was out driving. The revs were oscillating and then when I gave it some gas it stalled. I ended up leaving it in a parking lot for about 5 hours and it started right up and I drove home. I don't take it out of the garage anymore lol.

When I idle it in the garage it doesn't show any signs of stalling. When I shut it off in the garage it immediately wont start again.

[RSheiman]

The list of things you changed, heading indicates it was done for maintenance and not to pursue this problem. If so, swap in the old coil.

[DesktopDave]

I noted that you'd changed the CLT (DME temp sensor). Are you 100% sure it's the right part number? I've seen quite a few new "universal" CLT sensors for sale that appear to be listed incorrectly. Check the temp curve if you can, that's a vitally important sensor for hot starts.

As far as the "no spark when hot" problem, have you checked the little black 'code box' under the dash kick panel? Those have a habit of flaking out too, and will cause all sorts of random havoc.

[Griphish]

I finally had a chance to dig into the car a little bit more. Here is what I found.

• I tested the ignition switch by running a wire from the battery to the 15 terminal on the coil. Didn't help.

• I jumpered the 2 green wires on the OBC relay box to see if that was the issue. Still wouldn't start.

• I was able to hook up an oscilloscope to both of the flywheel sensors and I can see the peaks in the graph. Since these are new and the resistances are good I would think they can be ruled out as fine.

• I ordered a new CLT sensor to try and test the other new one I bought. I idled the car until it wouldn't start and then plugged in a new sensor that was cold. It still wouldn't start. Let me know if this test makes sense. Not sure if there is a time delay or something that locks out the car from starting for X amount of time.

Again, any input would be much appreciated. I still lean toward a standalone ECU, thinking that it may help me at least diagnose the car a little easier.

[DesktopDave]

You've done everything I'd have done, and more...that's a puzzling one.

The only thing that jumps pout at me is you didn't have spark when hot. Makes me think that the crank angle sensor is overheating, or the coil driver transistor is overheating or damaged. Did you test resistance on the sensors when the car is hot? These electrical components are highly sensitive to heat and voltage range. I'm not sure if that version of the DME got heat sinks around the driver transistors. Might be worth it to add one, if you have the space. Alternatively, run the DME with the cover off, maybe with a portable fan or a flex pipe to direct the A/C onto it, and see if that helps.

I'd also be pretty suspicious of the output stage of the DME. It runs various tightly regulated DC-DC output voltages, and those can become unstable if the reference voltage fluctuates. I'm guessing there's no spare DME to swap, as they're like hen's teeth.

Long shot guess...have you checked the alternator system voltage when it's warm? I've had very similar problems with solid-state ignition systems on various 80s bikes, one ended up being a bad voltage regulator. Admittedly, the permanent magnet alternators on bikes are far simpler than the BMW Bosch alt. Looks like it just uses a single transistor to regulate voltage, I'd guess it's a switch-mode shunting to ground. Might be a bad diode too. Any mystery parasitic drains?

There is a thermo-time circuit on a few older BMWs...some were even vac driven...but I think that system was eliminated by the time your car was built.

[RSheiman]

Last thing I think is the coil, these can crap out when hot.

[RSheiman]

Also, better hope it is not the ECU, impossible to find and if so, costly. You can send your unit to Programa and they will test and repair if needed.