I have a 98 GTP with 169,xxx. Lastnight it started missing when boosted, so i ran seafoam trough the tank and the middle vac line on top of the sc. I drove it home and let it sit over night.

Today i started it and it was missing at idle, and backfiring pretty bad. So i changed plugs(ngkTR6's) and wires(napa golds) and that didnt stop the problem so we tested the coil packs and when cyliner 6 and i think 3 were unplugged the backfiring stopped even under a load, so i replaced the top 2 coil packs and am still having the same miss. The only code it is throwing is p0300..so i cant be positive what cylinder it is.

Ive been reading the common causes of the p0300 code are cat or egr issues. But i have both deleted. Any tips?

Are you sure the wires are all the way on? Also did you clean up any and all corrosion where the coil packs mount?

We checked to make sure we had the firing order right probably 5 times. And yes i used 320 grit sandpaper to clean the corosion of the prongs the coils sit on

When I said check the wires I was meaning ensure they are pushed all the way on. Ive had wires that looked like they were all the way on but werent.