no-spark problem finally resolved - Page 3

Roger Shultz · Roger Shultz's Garage

Thank you for your comprehensive write up and how to do this.

I have a similar problem with my 1987 Carerra Targa where the car starts just fine, runs fine for as long as i want it to until I shut if off. It may re-fire immediately but after a minute or two it will not fire at all for many hours then the cycle repeats.

I've replaced the DME relay, checked resistance of the sensors, looked at them with a Tektronix scope where I see a clean 4 volt peak to peak sine wave. I've checked the coil resistances but the key is there is no action on pin 1 of the ECM when it won't start. I've open up the ECM and found no suspicious solder joints. This suggests the two sensors are having a subtle problem yet the scope traces are the same as well as the resistance before and after this happens.

So my question is, what happens between running fine and not starting? What is telling the ECM to inhibit the coil? Why does everything reset in a few hours and run like new?

If I had a breakout box for the ECM connector I might be able to spot something though I could probably open the ECM again and look at the leads going to the board but that seems risky.

I'm a bit too old to get under the car so I guess I'll farm out the sensor replacement because I don't know what else to do. Of course it could just be the ECM doing strange things!

Thanks again for this post.

3rd_gear_Ted · 3rd_gear_Ted's Garage

Very impressed by the table saw based X & Y coordinate measuring machine, sweet tool to have

NaijaBerry · 07-26-2022, 08:01 AM

I just did this a couple of days ago, the sensors were stuck in there hard and fast probably never changed, I had to hammer and chisel them out and grind the holes out. I used a plastic coke bottle top cut to size to gap the sensor, it was dead on 0.8mm, might be a fluke but it worked.

I changed the two sensors which are the same, the BMW sensor I bought is exactly the same and less than half price of the OEM. I changed the cylinder head heat sensor as a while I was there with a URO one which was also much cheaper, checked for spark and this car that been parked for 10 years in a muddy field fired right up!

Big thanks to the OP for this fantastic and detailed writeup��