I have a 1989 Firebird Fomula 305 TBI. This past spring I replaced my spark plugs and at one point it started misfiring and backfiring. I replaced one plug wire and it worked fine. I later decided to replace all my plug wires with some I bought at Autozone. Which turned up to be the wrong size (7mm instead of 8). With these wrong sized wires I noticed it misfire and backfire again. Loose spark plug, I tightened it and immediately the ICM goes (turned over no start). Unsure of the cause at first, I replace the coil, pickup coil, and ICM. Finally it starts again, still misfires and backfires. I replace wrong sized plug wires with correct size ACCELL plug wires. still misfires and backfires. I just replaced cap and rotor and checked timing. still misfires and backfires.

tl;dr Car misfires and backfires, replaced almost all ignition components.

 

7mm vs 8mm should have no effect. 7mm insulation should be plenty thick. Are you sure that all of your plug wires are correct? 1 8 4 3 6 5 7 2 in a clockwise formation?

 

Yes, the firing order is correct. The misfire is rather slight, it seems to come and go every second or so. This morning it was quite cold and my engine ran great for about 10 mins and then it was back to the backfire and misfire. I would think that would mean a plug wire is touching something hot or metal? I have made several attempts at routing them correctly.