Over the last couple of weeks, my car would experience fits of sputtering, spitting black smoke and hardly running. Sometimes it would die, sometimes I could keep it running. Sometimes it would last 5 seconds, sometimes 5 minutes, sometimes I couldn't start it for 8 hours. Finally I was able to log some sesssions via TunerPro. First I logged with the key on, but car not started. I'm seeing the TPS % jump from 0 to 30% for no reason. Battery and fuel pump voltage show 25 volts. IAT shows -40*.
Then I logged while (barely) keeping it running. TPS % stays at 0% no matter how much throttle I open. IAT still at -40*C. Battery and fuel pump voltage still 25 volts. Spark advance occasoinally jumps to 25,000* advanced (uhhh, ok).
First thought was the ECM is crapping out, but could it be a bad ground? I took a quick look at the wiring diagram and I see multiple grounds coming off the ECM, but I haven't tried to track them down yet. I think I'm barking up the right tree with either of these two theories, but what do you guys think?

 

If you suspect grounding, couldn't you try running another ground wire and grounding it somewhere close and see what effect that has? It kind of sounds to me like your ECM is totally whacked. ECM's are $114 at Car Quest, so it's not like it's terribly expensive. I think it would be a good place to start.

 

Thanks for the suggestion.... unfortunately, I didn't get the email notification. Looking at the wiring diagram showed several grounds coming off the ECM. Actuallly, I went down to my mechanic and he ordered me a reman ECM to try first and sure enough, that solved the problem. It looked like many of the values were getting doubled from my datalog (batt. voltage, fuel pump voltage, O2 voltage). It also provided a great time to install my prominator.

 

Yep. Mine fixed too. It was the ECM in my case as well.

 

Beats chasin' down a bad connection!

 

Well, the new computer DID seem to help. I went a good 20 miles, and it ran very smoothly. Then it started the same old crap again and guess what I found?

A 12 volt wire with a crimp connector that had a bad connection internally, causing arcing and burning the area crispy. So I guess on mine, I had to chase down bad connections and good chance that I found it. It was the power wire going to the computer, by the way.

That's okay, now I've got a spare computer.

 

That sucks you wasted the money on a new ECM, but at least in the end you got out cheaper that what the mechanic would have charged.

 

Oh yes. At $94/hour labor and a minimum of one hour diagnosis time, I would have easily paid the amount of money that I paid for that computer...IF I was lucky. Chances are, I would have paid at least $300.

I also improved my grounding, and verified all my engine bay connections, which probably isn't a bad idea to do periodically. I don't know what I'll do with the other computer, keep it as a spare I guess.

When I was driving my Dodge Shelby Charger, the computer was notoriously unreliable. I always kept a spare computer in the car at all times. The Chevy computer has been a zillion times more reliable though, so I probably won't ever need it.

In fact, I liked the IROCs and GTA's back in 1986 and considered buying one. However, I chose the $5000 cheaper option and bought the turbo-Dodge. So which car is still running? The Dodge I still have, but it's in storage and doesn't run. The IROC has a few hundred thousand miles on the chassis and a new engine, and still runs great.
Seems to me that it would have been worth 5 grand more to buy the reliability of a GM product!!!