Car's dying after header install....
 

OK, this is not my first post on the subject, but here goes.... after I got my Pacesetter LTs, ORY, rear O2 sims installed my car ran like crap. Backfired and stumbled. I checked all the wires and identified a cracked plug. Replaced all the plugs anyway and the car still misfires, backfires, and stumbles, almost worse than it did with the cracked plug. What the hell is going on??? I'm getting really pissed since this is my daily driver.

Have you checked the coolant for air bubbles?

Since the plugs are new (tho I have seen new ones be bad before) the problem is probably the wires
1st use an ohm meter and test the wires for excessive resistance (should be around 100 ohms) replace if needed
2nd make sure the wires are seated all the way, it'll be one click on the plug, and 2 clicks on the coils (use some dielectric grease to allow the boots to slide on easily)
Test to see what cyl isn't firing, drop a little water on the header tube (near the head) and see if it evaporates, if it doesn't that's the dead cyl

Did it happen as soon as they were put on, or a few days later? My car ran great for about 2 days then backfired, ran crappy, stalled....my air fuel was really rich and it just wouldn't run right. Put it on a dyno and adjusted A/F ratio and was all good.

recheck everything again(i know its stupid) it took me 3 checks to find a hairline crack in one of my plugs..i felt like an idiot