I've been doing a lot of reading on the forums, trying to sort out a miss on my 100K mile 1997 C5. Here are the symptoms, and what I have done so far.

I started in on this because of a miss or sputter under load that was most noticeable under 2k rpms in 4th, 5th, or 6th gear. Seemed to me like plug wires, so I ordered the GM red wires everyone is talking about.

So, the wires come in the mail, I am removing the old ones, and I cracked the #7 coil pack insulator. I wound up epoxying it back together, and checked for arcing and found none. I also pulled the wire while running and the miss got worse. (yes i know this is not ideal, my next step is replacing this if I don't come up with any other ideas, ). Then I installed the GM red performance wires. Tested them out, still had the same problem. I checked them all again to see that they clicked in properly, and all is good. Test drove again, same problem.

So then I replaced the spark plugs. (old ones looked original) I went with the TR55 copper plugs(Autozone #3951) as recommended on several forums. I left the gap at the stock 0.052". Installed them. Tested, and now the miss goes from a load related miss to a constant miss, seen at idle to all the way up, seems like a cylinder is totally out, way down on power.

So I check the spark plug connections again, no issues. I've had problems with brand new plug wires before, so I picked the best looking wire from my old set, and installed it one at a time on all the different cylinders. Made no difference.

Next I decided I had better check the plugs, so I pulled them all out. None looked really different at all, no cracks anywhere. The #6 plug was the only one that looked different at all (had a 1/8" dia black smudge on the inside insulator, don't know the cause, but I replaced the plug anyway).

I still have the same problem of the constant miss. The computer is not throwing any codes, not even on an obd 2 scanner. I've heard mentionings of the P0300 code not coming up for a single cylinder misfire, and that you need to use a scanner to detect individual cylinder misfires. But like I said nothing comes up on the autozone scanner.

What do you guys think?

 

I'd disconnect the maf to put it in limp mode to see if the miss goes away. If it doesn't go away, you'll know it's not sensor related. Do you know somebody that has a another coil pack you can try? I'd be very suspicious of that one broken coil pack.

 

Here's what I would do ...

First to figure out which cylinder .... pull plug wire #1 off the coil. Start engine. If miss is WORSE then cylinder #1 is good. Replace wire on cll #1 and pull wire for cyl #2 .... continue to do this until you find the cylinder where pulling the wire MAKES NO DIFFERENCE .... that is your "bad" cylinder.

Either you have a spark problem, or a fuel problem (assuming no mechanical issues like a bent valve). One way to see if it is the coil is to swap the "bad" coil to another cylinder. Does the problem move with the coil ??? If the miss moves ... bad coil. If it stays, then do the same thing with fuel injectors, swap two cylinders injectors. Did the problem move ... if so ... bad injector. If it stayed with that cylindeer ... having eliminated spark and fuel ... time to pull the valve cover and see what other possible issues could cause this.

 

As others suggested, that one coil sounds suspect. While you are checking things, try looking at the PCV tube that connects it to the intake. Specifically, look at the bottom of that tube where you normally can't see. Had mine develop a large hole there that wasn't apparent.

 

I didnt see what it does under WOT? Does it clear up? I had a car in the past that I was having low RPM issues with and it turned out to be an O2 sensor. Never threw a code. Some ECMs are a little less responsive(read... dumber) than others, particularly on an earlier Gen III ECM.
My advise would be follow what Black Z06 said, removing one wire at a time to see if your miss is due to A SPECIFIC CYLINDER. If it is not, I would advise replacing your O2 sensors. Or at least, get a buddy with a more broad range OBDII reader so you can compare bank trims and figure which O2.

 

Check the O2 sensors. I chased a similar problem and finally found it.