'88 M6 Hot Start Issues

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Griphish
Posts: 15
Joined: Sat Nov 21, 2020 3:56 am
Location: Charlotte NC

'88 M6 Hot Start Issues

Post by 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 fuel pump 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
Any help on this would be much appreciated!

Cheers!
davrianmad
Posts: 19
Joined: Mon Mar 28, 2022 3:33 pm
Location: South Lincolnshire

Re: '88 M6 Hot Start Issues

Post by davrianmad »

If you are not getting a spark, I am guessing that will be the problem, I would check the coil as they can cause problems when they get hot, distributor cap and rotor arm.
BMW M635CSi 1985 0760152 RHD B512OPH
BMW 2002tii 1974
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Griphish
Posts: 15
Joined: Sat Nov 21, 2020 3:56 am
Location: Charlotte NC

Re: '88 M6 Hot Start Issues

Post by Griphish »

Everything I can test on the coil points to it being good. When I test for spark on the wire from the coil to the distributer I get no spark which makes me think that it is the DME.

Now, I have also tried to test if it is the DME getting too hot. I idled the car until it was hot and wouldn't start. Then I removed the DME and put it in the freezer for 10 minutes. after it was cold I reinstalled the DME and the car still wouldn't start. Not sure if that is a good test of the DME or not.
Griphish
Posts: 15
Joined: Sat Nov 21, 2020 3:56 am
Location: Charlotte NC

Re: '88 M6 Hot Start Issues

Post by 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.
TimU
Posts: 280
Joined: Tue Dec 14, 2021 2:16 am
Location: Sydney

Re: '88 M6 Hot Start Issues

Post by TimU »

Seen this?
viewtopic.php?t=33245

Sounds like you've worked through most of the steps in the list, but it's worth double-checking with a fresh set of eyes.

Also here:
https://hpsimotorsports.com/blogs/tech- ... ch-article
04/1986 635csi Euro spec
Australian Delivered, RHD
TimU
Posts: 280
Joined: Tue Dec 14, 2021 2:16 am
Location: Sydney

Re: '88 M6 Hot Start Issues

Post by TimU »

Griphish wrote: Thu Oct 13, 2022 11:05 pm I ordered a new CLT sensor
What's a CLT sensor?

Other things to try:
(My experience is with the M30, so apologies if these are different on the M88)

- Jump bypass the Main Relay. Just to totally rule out a dicky relay.

- Test/Replace the Coolant Temp Sensor (CTS) (Blue Head at the thermostat housing on the M30, not sure about yours)
Blue temp sensor is responsible for coolant temperature information to the computer for proper engine performance.

- Test/Replace the Temperature-Time Switch (TTS)
Brown thermo-time switch is responsible for cold start valve operation, it reads the coolant temperature and if it's low enough then the cold start valve will fire. A faulty TTS could cause the CSV to fire when it shouldn't.

- Disable the Cold Start Injector (just unplug it)
04/1986 635csi Euro spec
Australian Delivered, RHD
Griphish
Posts: 15
Joined: Sat Nov 21, 2020 3:56 am
Location: Charlotte NC

Re: '88 M6 Hot Start Issues

Post by Griphish »

TimU wrote: Fri Oct 14, 2022 2:04 am Seen this?
viewtopic.php?t=33245

Sounds like you've worked through most of the steps in the list, but it's worth double-checking with a fresh set of eyes.

Also here:
https://hpsimotorsports.com/blogs/tech- ... ch-article


Thanks for the links! I'll take a look through and see if there is anything I missed.


TimU wrote: Fri Oct 14, 2022 3:10 am
Griphish wrote: Thu Oct 13, 2022 11:05 pm I ordered a new CLT sensor
What's a CLT sensor?

Other things to try:
(My experience is with the M30, so apologies if these are different on the M88)

- Jump bypass the Main Relay. Just to totally rule out a dicky relay.

- Test/Replace the Coolant Temp Sensor (CTS) (Blue Head at the thermostat housing on the M30, not sure about yours)
Blue temp sensor is responsible for coolant temperature information to the computer for proper engine performance.

- Test/Replace the Temperature-Time Switch (TTS)
Brown thermo-time switch is responsible for cold start valve operation, it reads the coolant temperature and if it's low enough then the cold start valve will fire. A faulty TTS could cause the CSV to fire when it shouldn't.

- Disable the Cold Start Injector (just unplug it)

CLT is what I'm calling the coolant temp sensor. I've seen it called different terms, not sure if there is an actual difference or if its CooLant Temperature sensor vs Coolant Temperature Sensor.

I've jumpered the main relay and installed a new one and both attempts didn't help.

My car doesn't have a cold start injector system because it has Independent Throttle Bodies. There's also only 2 temp switches for the engine. 1 for the DME and 1 for the instrument cluster. There's 2 more temp sensors on the radiator but they are tied into the HVAC system.

Thanks so much for the help everyone. As I mentioned at the beginning of the thread, I'm still leaning toward a standalone ECU since finding a working M6 DME is extremely rare. To me the ability to have some diagnostic ability and tune the engine to run on Alpha-N might be worth the extra cash. I don't want to bastardize the car too much though since it's relatively rare in the M6 form.

-Steve
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