P0300 is random/multiple misfire. This indicates the failure is within a primary ignition component like the ICM or PCM. Heat is the worst enemy of any electrical component, especially the ICM. There's supposed to be heat sink lube between the coil packs and the ICM to shield the ICM from heat generated by the coil packs. ICM's degrade quickly without that insulation. I'm not suggesting the fire alone melted out the lube and/or damaged the ICM, but age plays a factor too and it might have been just enough to push it over the edge. I wouldn't kick yourself too hard if you have to take it to a shop. You've given the a car a second life for minimal $. Sometimes two heads are better than one.

Good luck and please keep us posted Steve

 

Well, today it ran great. 60 mile round trip again and not a single misfire.
I think as soon as I get a chance I am going to head back to the junk yard and pull some electronic parts. I am going to replace the ICM. I am also going to replace sensors on the left side of the engine near where the fire was. The EGR did have heat damage to the plug, and the fire was not far from some of the other sensors.

 

Also possible a chafed wire is grounding out.

 

That is possible. Sunday night it was running rough again. I went out in the dark and looked under the hood. I wanted to watch the plug wires in the dark to see if I could see any signs of arcing or flashing from bad plug wires. I also listened closely for vacuum leaks, and wiggled the vacuum lines and electrical wiring in different places to see if I could find any shorts.
Since then it has been running fine. That makes me wonder if it does have a bad wire somewhere. I looked everything over carefully for heat damage to wires back when I repaired the intake problem, but I wonder if I missed something and either have a short or a plug that is not staying tight.
A few years ago my wifes Expedition was giving me trouble. When it was warmed up not under a load it would stumble. Not a hard enough miss to pinpoint, it would act like one cylinder missed one or two firing cycles. I chased that problem for 3 years, but it did not do it often enough to pinpoint the problem. After 3 years it really started giving me problems. The OD light flashing, not going into overdrive like it should, missing, all kinds of problems. After another couple of months of chasing this problem I finally found the problem. The wiring harness going down to the transmission shift position range sensor had melted to the exhaust manifold on the passenger side. I had tested every wire in the harness, but when it was sitting parked the wire was not shorting. When it got warmed up it softened the wiring enough that the wind blew it back onto the manifold. SO, next time I can get into the shop I am going to spend some time chasing wires and examining the plugs to the injectors and the sensors. I did have to replace the connectors to 2 of the injectors.

 

I am getting so sick of this car! I am not a Chevy fan anyway, but this is a nice car and my wife loves it.
Anyway, still running bad, especially when it is warm. There were a couple of days that it ran great, then started missing again. Now it misses almost all the time, low speeds high speeds and under load or idling. It is really noticeable under a load of course.
I pulled the codes again the other day and got the P0300 and P0131 again. Cleared them and checked the next day, only getting the P0300.
It is missing so bad we cant hardly drive it around town. Tonight I checked and NO CODES!
As much as I hate the idea I think it is time to go to a shop and have someone else work on it. I have not had anyone else put their dirty hands on any of my cars in over 30 years, but the only other idea I have is to just start hanging parts until I happen to find the right one.

 

Which engine does the car have and what has been done so far to resolve these issues?

 

It has the 3.8.
So far all I have done is replace coil pack #1.
When it first started acting up it was after a 50 mile drive, it ran rough for a few minutes. Shut it off for a couple of hours and it ran fine for a couple of days. Every few days it would miss especially when it was warm outside or after a longer drive. Engine code then was p0301, misfire on cylinder 1. Cleared code and ran it, after a few weeks of missing off and on again misfire on cylinder 1. Checked spark plug and wire, they were good. One check of the codes I got po304, miss cylinder 4. It had been my experience (with Fords) that a problem that comes and goes at random like that it is often electrical.
New coil did not help.
Now I am not sure what to do to fix the problem. After the coil I got p0300 and p0131, random misfire and upstream O2 sensor. Cleared the codes and rechecked a few times, always the 300 and 131. So, I was tryin to decide if the 131 was a cause or a symptom. Now I am only getting the p0300, not the 131. Good thing I didn't change the O2 sensor. The problem has also gotten worse, it has a miss when hot or cold and at all power ranges. It is a random hard miss, meaning it will run smooth for a few seconds then miss then run strong then miss, but it is doing it all of the time now.

 

Earlier in this thread someone suggested the ignition control module could be going bad, so I was thinking of changing that. But I don't want to just start hanging parts until I find the right one.

 

 

Thanks, that is a lot of good information!