I need some help please with a recurring problem.

Here's the car and the problem. It's a 1994 with 105,000 miles on it that is used for autocross only. Engine is stock (due to class restrictions) as far as I know with the following minor mods: Exotic Muscle LT headers, Random Tech cats, O2 simulator, Corsa cat back, Taylor spiro pros, and a mild tune by PCM for less.

The problem is as follows, after allowing the car to get up to operating temperature it runs great. We can make about 5 to 8 runs and then it starts backfiring, bogs really bad at launch, and blows black smoke out the tailpipes. This seems to go away when on the track and you get it up to WOT. Then it comes back when you ease off the throttle. If we shut it down and let it sit we can run again the next day with the same results, 5 to 8 runs that are great and then backfire and bog.

No changes were made to the car when this started. The mods were done months before this started. I have replaced the fuel filter and the fuel pump. We tried reading the codes but nothing shows up. I have checked the plug wires and they are snug.

One kind of strange issue is that when I disconnected the battery for about 5 minutes when it starts doing this when I reconnect the battery we can typically make about 2 runs before it starts up again. As said earlier, we then have to let it sit overnight to run without blowing black smoke.

My guess is something is causing it to run rich, but what? Any thoughts and suggestions would be welcome.

 

I'd take a look at the coolant and IAT sensors, one could be reading incorrectly without setting a code as long as the reading is within range of accepted parameters.

 

Have you check the O2 sensors?

toptech has a good suggestions.

 

Never heard of 02 sims for our 94s.

 

It replaces the stock O2 sensor on the header collector.

 

What I'm saying is I've never seen a place that sells 02 sims for the 94 LT-1. Where did you get it? Maybe you shouldn't be running a sim?

I run heated 02 sensors with my headers no problems. The 3rd sensor isn't used, is that the one you're talking about?

 

Merle at Exotic Muscle sold it to me as part of a complete install kit when I bought the headers. He said it needed to be used to keep the ECM working properly.

I was thinking it may have gone bad but don't know how to check it, plus if it did go bad why would it "reset" after a while? The headers have been on for almost a year with no issues. This problem just started about two months ago.

 

EGR valve going bad?

 

Have you had the ignition control module tested? With what youre saying this is what im thinking is the probable cause. Since the car runs good til it gets hot, id look towards ICM. Its mounted on the front surface of the right hand head, along with the coil. When it gets old and tired, it can cause problems when it gets hot.

I wouldnt think the O2 sensors are the cause, but then its not out of the question. If it is related to O2 sensors, it would more than likely happen the second the ECM goes into closed loop mode. They would be more than hot enough by this time.

The fact that you can make 5-8 passes before this happens leads me to believe you are well and truly up to working temp, and in closed loop for most if not all of this time, which i reckon rules out any relation with O2 sensors (especially since they are heated anyway)

As a test unplug the O2s and run without them and see if it still does it. If it still does, its not them. As another test, start the car from cold, make sure the issue is not present, then let it sit and ilde til its been hot for a fair while and everything in the engine bay has had plenty of time to heat right up. Then take it for a run, if it does it after just idling for about 20-30 minutes then id certainly look to the ICM.

You dont need a sim, you can disconnect the 3rd O2 without a code or a problem.
The car came from the factory with 3 HEATED O2 sensors fitted, and ALL of them do something. The 3rd sensor IS used, and its not there for a show and tell. But if you remove it the car will run fine without it and no codes will result. Its not required, but it certainly is used. (since when have you known GM to give you anything they didnt need to?)

 

As posted above I found out that I didn't need the simulator. OK, well so he sold me extra stuff I didn't need. Live and learn.

We had it up on the lift last night to look around underneath and think we found the problem. It appears that the connector for the O2 on the right said had slipped and was making light contact with the header tube. Didn't completely melt it but the jackets on the wires on one side of the connector were gone and the bare copper wires appeared to be making contact. Or at least they would when hot and running.

I fixed that and secured the wires and then let the car idle for 20 minutes then drove it for another 20. I did several aggressive launches, which was when the problem was most noticeable, and it didn't seem to backfire at all. I cleaned the tailpipes before taking it out and when I got back they were still clean. When it was doing this if I cleaned the pipes and ran the car they would be black in minutes.

We'll see. Hopefully this fixed it. Thanks for everyone's input.

 

Glad you found it, sounds like it should run like a champ again. Can't believe that guy sold you useless junk, but as you said, live and learn what parts gm installed that the car doesn't need.

 

Thats a new twist to the story which leads me to a new question, with this issue on the right bank O2 sensor circuit, is/was the issue more noticed on the right hand bank?
Shouldve, it certainly cant have hurt it!!

 

It blew black sooty smoke from both sides. Maybe it went into a limp home mode and ran everything on the rich side thinking it was running too hot.