Car had a slight misfire at idle before changing plugs & wires. I looked at the wires from underneath the car and sure enough on the passenger side the wires were chewed up from the belt. I ordered the house brand Summit Racing wires p/n SUM-867825R and for the plugs i got NGK TR55GP. I gaped the plugs to the factory spec. Once i put everything back together i had a bad misfire. It wasn't noticeable at first, it seemed like i still had a slight misfire at idle. However when i started driving it the misfire was really bad in 5th and 6th gear in lower RPMs. After driving the car a little more the problem got a little worse, now misfiring in most gears unless the RPMs are 3k or above. I also purchased a new coil to go along with the plugs & wires so i threw that on hoping there might be some relief in sight. No change with the new coil either.

So far i have gone back and traced the wires to make sure they are in their proper order & re-seated the wires. They all fit well on the plug side but they seemed to not snap in place Opti side; some of the wires even wanted to slip back off the Opti. To fix this I gave the connector a slight squeeze to tighten them up then re-seated the wire to the Opti. This time it seemed like the wires snapped in place like they should. The misfire is still present after doing this. Now I am thinking the wires might be bad so tomorrow or Wednesday i'm going to do a resistance test on each wire. I already tested the coil wire, it was 360 Ohms. Am I heading in the right direction with my troubleshooting or is there something more obvious i'm missing? Also, assuming the wires test fine what should I look at next?

I tested the resistance on the wires tonight and they were all about 241 ohms per foot of wire, give or take a little. Given the research i've done this seems to be acceptable for an aftermarket wire. When i removed the wire from the Opti they still just didn't have that feel like the factory wires do, so i'm going to return them and switch to a different brand of wire. I did verify for a third time though that they were in fact going to the correct cylinders so at this point i think it's a fitting issue on the Opti side of the wire. I should have the new wires this weekend so hopefully Saturday I can get them on and test it out.

I'm thinking this time if the misfire is still present i'll double check the plug gap and verify that there are no cracks in the ceramic insulator. If I still have a misfire after the second set of wires and checking the plugs I will move on to testing the Opti. If anyone has any ideas or feedback it would be much appreciated.

There several things that could cause the miss fire...I have seen a bad connection at the opti
oxygen , crank , cam sensor , fuel pressure ..
have you checked for trouble codes?
there are plenty of people on here that could give you better advice than me but until they do
Use the search box and look for threads that seem to be related. good luck

Sorry, i should have mentioned that i did check for trouble codes and got a p0300 one time. After i checked the connections the first time and replaced the coil i cleared the code and drove it some more. It was still missing but there weren't any codes after that. I must admit though i just use a free program on my laptop that won't pull the advanced gm data, so i can't tell the number of misfires per cylinder or anything fancy so maybe there's something i'm missing there.

I've been searching through the forum and found some people that had bad wires, some had bad Opti's, some were due to crossing the wires and others were cracked plugs, bad coil or bad ICM. My coil and ICM are both new; the ICM is only about a month old. Some other searches turned up fuel related issues, but i just replaced the pump, filter & regulator about 4 months ago so the only thing left in that area i believe would be the injectors. However, since i just changed the plugs and wires i'm leaning towards those.

Let's try moving this the LT1 Based Engine Tech...... a lot more traffic there.

I think you are on the right track since it got worse after changing the wires. You may have a bad one out of the box.

I recently developed a misfire under light load in 6th gear. At night, I could see high voltage leaking from a few of the wires and from around the ignition coil. I think the real problem was the wires, but since my coil and ignition module were ~16 years old I decide to replace them with the wires.

My plugs are new and the opti is a new GM unit that has only around 5K miles on it(~4 years old).

I feel the real problem was the plug wires. I was running some Moroso blue 7.5mm(I think that was the size...could have been 8mm) plug wires and they are decent wires. However, the boots are the most hateful things I have ever tried to put on a plug. They are so tight that you can't feel them "click" when getting them on the plug....and in the tight confines of the 4th gen F-body engine bay that is a problem.

Anyway, I went with ACDelco wires from Rockauto.com. The are a little expensive compared to an Autozone special, but they are really good plug wires and easy to snap on to a plug.

Replaced the ignition coil, module, and plug wires. No more misfire. I am running long tubes so it pays to be careful about wire routing. I'm using the stock GM plug wiring looms(metal) with no problem. Just got to position them away from the primaries. Also do your best not to manhandle the plug wires. Don't want to "generate" a problem by hurting one of them.

Put the new wires on today, went with Taylor Spiro-Pro. They fit very well, and both ends snapped on like they should. I believe that was the problem was with the Summit Racing house brand wires; the end that connects to the Opti had a very poor fit. Even after giving them a little squeeze they still fit poorly. So the test drive went well and all misfiring is gone.

Now it looks like I'll be doing a water pump next; noticed a small drip coming from the weep hole on the bottom of the pump while I was putting the car back together today. Always seems like the old parts get jealous of the new ones and break.