OK, I'll try to be specific and brief as possible:
'95 LT1 Camaro, new 383 LT4 conversion.
New Accell opti, new O2's, most sensors replaced with new.
New Magnacore wires, new NGK plugs (TR6 IIRC)

Car fired up fine and ran great for about 500 miles. Last time I drove it a miss gradually developed over about 3-4 miles (5 minutes or so) of city driving. Identified #3 as the main miss, however #7 also seems to miss while cold, but both improve as car warms up (at idle for 4-5 minutes), #7 improves more than #3. Compression on 3 is fine, have not checked compression on 7. No SES light, but I have not scanned it, I don't have a scanner. I identified the bad cylinders using the "which header tube is hot" method. #3 stays pretty much cool to the touch even after 2-3 minutes of idling, but #7 eventually gets too hot to touch, but much slower than the other tubes. I also used an old school timing light to check to see if the cylinder is firing. For example, putting the timing light on #1 shows a steady flash, but #3 shows an intermittent miss, 7-8 flashes followed by 2-3 missed, more flashes, then a few more missed, etc. Have not yet tried #7 with that method, and also need to verify for certain that the problem gets better as the engine warms, but I am pretty sure that is the case. The car also has an Accell 300+ ignition, I bypassed it to make sure it is not the problem and ran it stock, no change in the problem.

Finally, when this first happened I crawled underneath the car to check things out, and found a broken ground wire, the small black wire on the passenger side just under the exhaust which I believe is the ground from the ECM. Thinking I had found the problem, I re-attached the wire with no improvement noted. Could this still be the problem? Is there some "reset" I have to do for the computer? Also, I was thinking perhaps the O2 sensor on the drivers side may be fouled since the problem did seem to develop gradually, but if I'm understanding things right the O2 sensor will only control the fuel trim, not the spark, so that wouldn't be the problem.

Any ideas which way I should procede? Anything I may have overlooked? Help is MUCH appreciated, I'm hoping to be doing a lot of driving in this car for summer...it's been waaaay too long since I've been able to enjoy it!

 

First guess would be the Accel Opti. Many people have had them fail right out of the box.

Did you run it in the dark, and look for arcing on the plug wires? Are the coil and IC module new? What did you gap the plugs to?

O2 sensors can cause misfires, if they get the A/F ratio screwed up bad enough. A small miss turns into a big miss, when the air from the missing cylinder fools the O2 sensor into thinking the engine is running lean, and it starts pouring fuel into that bank. An exhaust leak before the O2 sensor will have the same effect.

Have you switched the injectors to see if the miss follows the #3 injector? Have you put a 'noid light on the injectors to make sure they are firing?

You really need access to a scanner. It makes no sense to dump all that money into and engine, and then get cheap and not buy the tools you need to keep it running right.

 

Thanks Fred, I was hoping you'd jump in here.
I hear ya on the scanner, buying one this weekend. I did check it in the dark, no arcing anywhere. The coil and IC are not new. Plugs are gapped at .042 IIRC, but I'll check. Wouldn't those things give more of a random miss across all cylinders? My problem is confined specifically to #3 and #7, far as I can tell. I have a noid light, I'll check those injectors to be sure they're firing.

When I get the scanner, where should I start?

 

Are the wires for #3 and #7 crossfiring? That arc would probably not be visible from the top of the engine, depending on how you ran the wires.

 

I was thinking about that, I did check from underneath and also unclipped the wires to seperate them further. I know those two aren't sequential in the firing order, but they are next to each other on the cap....makes me wonder if there's a problem in the opti. I'm going to Autozone tomorrow to get a scanner, got any recommendations? Once I have it, what should I be looking for? The IC is my original stock unit from '95, but from what I've seen they tend to be heat-sensitive when they're bad, my problem seems to improve as the car warms. Probably wouldn't hurt to replace it anyways. I can also swap one of my old O2 sensors back onto that side to see if there's a difference. Could there be stored codes without a SES light? I thought if it threw a code, it would automatically trigger the light, or am I wrong?

 

You're going to have to be careful about buying a scan tool for a '95. I'm sure you know it's OBDI with an OBDII port. Be sure when you buy the tool it has the proper cable to connect to the '95. I'm not sure Autozone even sells one that will work. However, there is a way to make a conventional OBDI scan tool work, and that's to jumper the connections to the scan tool. Shoebox has the directions on his site.

I'd buy one that can do both OBDI and OBDII.

 

Not all codes turn the SES light. Most codes do turn on the SES light, and the code is stored. If the problem goes away, the SES light goes out, but the codes remain in memory until the vehicle completes a fairly large number of "drive cycles" without the problem recurring.

One problem with the Optis is the breakdown of the dielctric strength of the epoxy that is poured in the cap, to cover the conductors that route the individual cap "buttons" to the correct plug wire tower on the sides of the Opti. MSD claims their epoxy is more resistant to dielectric strength breakdown. I doubt anyone knows if that is actually true.

Is it possible you have a cracked insulator on the plug for #3 or #7? Might be worth pulling those plugs to check.

 

Moparman: How about the Actron 9190 kit? It seems fairly complete, works on OBD 1+2, and includes several different cables. I'm assuming I would just use the included OBD2 cable on mine, even though it's OBD1 on my '95 as you noted. Auto Zone is almost $500 for that one, I saw it on ebay for $290....much better

Fred: I'll check those plugs. If the insulator is cracked though, wouldn't my timing light still flash continuously, or does a cracked insulator break the ground connection? The intermittent flashes are puzzling me. I thought perhaps that was not an accurate enough way to check the spark (sort of like using a 'noid checker on an injector, I guess) but the cylinders that do not exhibit the problem show a nice, steady flash, no skips at all. #3 was an obvious intermittent flash, and I'll be checking all of them this weekend to make sure.

Any thoughts on the IC module? Mine's 15 years and 60K miles old, can it be intermittent on just a cylinder or two, or would it be more pronounced across all cylinders? Same goes for the coil, can it do that also? I've never seen a "slightly working" coil, all the ones I've seen go bad were totally dead.

 

Look on Amazon. Search for scan tools. They have several different brands. Actron and Autoxray are made by the same company. You might look at the Autoxray 6000. I seem to remember it's around $290 and has all the cables and has the enhanced OBDII capability. It will have the proper cable for the '95. I'd guess the Actron also has the proper cable. A regular OBDII cable won't work.

 

:update:
have checked all cylinders on drivers' side. Compression is fine on all, "timing light" method shows BAD missing on #3, Moderate missing on #7, occasional miss on #5.
Swapped in new plug on each of those cylinders, no change
Swapped in old but good O2 sensor on drivers' side, no change.
'Noid light on injector wires show pulses, and #3 plug was wet with fuel when removed after idling for 30-40 seconds

I'm checking the passenger side tomorrow. I'm really beginning to lean towards the Accell opti being bad. When I built this engine, those units were still pretty new and had not acquired the reputation they have now (Thanks for leading me in that direction Fred). I just read several threads about those- the search button is my friend- and a lot of them mention problems that seem rather similar. I've ordered a scanner, and will probably order an OEM opti and the IC module while I'm at it...my ICM is probably still good, but I'll consider it "preventative maintenance". Checked eBay for "thepartsladi", had to send them an email for an opti, they don't have any currently listed. Anybody know of any other good opti sources out there?