I need some second opinions on a stumble that I've started having. I have a couple of things in mind, but they could be $$ (and I don't want to throw $$ where it doesn't help....even if it's $10)

Stumble only happens 1500-2500 rpm. Usually (but doesn't always happen) happens after a cold start in the AM and 6-7 miles of steady cruising at 2K or so. It will stumble for maybe 5-10 seconds, and will not do it again that day. If it does do it again, it's only 1 mile further down the road.

I've hooked Autotap to it. I have misfires on cyl 4 and 8. No knock retard. IAC counts are normal. MAP is normal. Fuel trims are normal. I think it's either plug wires (70K miles old) or opti (carbon tracking maybe?). I have hit puddles in the past and encountered stumbling too. I am 99% sure that it is one of the two, just don't want to go through the trouble of installing (or buying) one and not fixing it. I would hope wires since they are they cheaper fix (but a pain to put on). Its strange that it only happens in the AM and usually in the same spot (on my way to work). I would tend to think that arcing problems would be at all rpms, all temps, and not only once or twice and not anymore that day.

ideas?

 

Since nobody chimed in maybe it's the heat shields trying to find a ground on those two plugs.I dunno.Or vise versa.Beer time here.Hope this helps.

 

LT1's are notorious for intermitant and spasmotic misses. Maddening and expensive problems eventually turn some owners off on this erstwhile good engine. The two wires you mentioned and the coil wire should be replaced and see if that helps. If not, look to the opti gradually going bad.

Another area that should be checked is the two harnesses to the opti. One is about a foot long and directly hooks into the top of the opti...the other connects to it and then connects to the ECM. Check them out with a spy glass and a light, or a used replacement or with a shop that knows this engine...most everyday places will simply throw expensive parts at it and perhaps never, completely fix the problem. Finally, the two temp gauges...one to the engine oil and the other to the water pump area. The connections, too, need to be secured. These affect the misss at different engine temps. All of us who have LT1's experience these from time to time and it takes real thinking and time to work them out.https://www.camaroz28.com/forums/ima...lies/smile.gif

 

I still find it odd that cylinder 4 and 8 are the ones with misfires since they are back to back in the firing order. I was thinking that if were something common to all cylinders, like a coil wire, it would misfire on all cylinders (which it isn't). I have logged the data multiple times and I am seeing the same readings. I still have the original opti that was working fine when I changed it when I did the heads and cam. I guess that I will start with the cheapest and change the wires.

The rpm range at which the cylinder misfire occurs will bug me until I can explain it.

 

after yo replace the wires if it does not help...the opti must be checked. Let us know how u eventually fix it.

 

Are those the only two cylinders with misfires?

Two cylinders might indicate cross-firing on the wires, but those wires would not be adjacent to each other in the stock routing. Within the Opti cap, the epoxy filler loses its dielectric strength, but the internal conductors in the cap for those two cylinders do not pass near each other.

That would lead to your one possibility of carbon tracking in the cap, or to an Opti bearing problem that allows excessive play when the rotor passes the 4/8 buttons.

 

Pretty much. The other cylinders might have had 1or 2 misfires, but 4 and 8 had 10 times more according to the misfire history for each cylinder. The plugs had about 80k on them when I logged the data. I changed them this weekend, still have the stumble. I figure the other cylinders might have had a few misfires from the old plugs. I though about looking into the Bailey LTCC system, - but that isn't cheap either.

Did MSD ever get the problems solved with their opti?
Anyone try the GMPP opti?

 

The MSD has reached the point where it is probably as reliable as the stock GM part. But the price is high and the "adjustable timing" is still an issue (no defined "0" point). The GM Performance Parts Opti is a factory GM Opti.

 

After I changed the plugs (and had to disconnect the battery), I decided to recheck the misfire counts again. I decided to just add the short term fuel trims to the list. First thing I noticed was the difference between the left and right STFT. The left side was +/-10% while the right was +10% to + 50%. The right O2 seemed really sluggish in its rich/lean transitions. I guess it didn't show up last time due to the PCM adjusting to a gradual deterioration of the O2 sensor. I though I looked at ask the data the first time, but maybe I just overlooked it. I happened to keep an old pair of O2 sensor that I had changed a few years ago as a matter of routine maintenance. I changed it out and looked at the data. It looks better as both sides are much closer STFT. Won't know for sure until tomorrow morning, but I think that would explain the temperature effects and the timing of the stumble. Once the engine got real warm, it wouldn't stumble. I was much more likely to do it on cold days. It could also explain why cyl 4 and 8 are misfiring if there is that much fuel in the cylinder (since they are the 1st two on the passenger in the firing order). That would be a cheaper solution anyway.

 

If you started it after the battery cable was pulled, the LTFT's were reset to 128 and the right side was using the STFT's to jack up the fuel being supplied, until the LTFT's could adjust. It takes time - that's the "learning". As the LTFT reaches the required level, the STFT's will then be oscilating on either side of 128. The issue is why is the right side adding so much fuel, whether its adding it with the STFT's or with the LTFT's. Same result either way. When you changed the O2 sensor, did you again reset the PCM, or did you just let it continue ratcheting up the LTFT's?

 

I did not reset the PCM YET. I was going to do it this morning to completely duplicate the conditions from yesterday. The main thing I noticed last night (without resetting the PCM) was the speed at which the Rich/Lean transitions were occurring. This old sensor I saved seems to be changing much quicker. I'll see how it goes in about an hour.

 

The new (old) O2 sensor took a couple of miles before it started switching rich/lean as fast as the left sensor, but it did work. No stumble this morning. After resetting the PCM, the STFT's were the same for both sides (withing 4-5% of each other). No stumbles or backfires like yesterday. I'm gonna run with this sensor for a couple of more days before I order new O2's just to make sure.

 

I guess it doesn't really matter now. On they way home yesterday, I got caught in one of the typical summer thunderstorms with torrential rain, and I hit a deep puddle that I couldn't see until it was too late. Opti got water in it, so the car died, and I had to have it towed home. Since I am not going through that again, I'll be replacing the opti, wires, water pump this weekend and the O2's next weekend. So much for a cheap fix....