Ive got a 99 w/ 4.2 the driver side header has been cracked badly for awhile but the truck still ran fine. I finally got around to changing it out. Afterwards the truck started ROUGH and runs HORRIBLY. Lots of popping/back firing noise, wont go over 25 even when floored and horrible acceleration. I noticed the cat was glowing red hot as well after about 10 minutes of trying to drive it. Anyone else had this problem or got an idea as to what it could be? Seems like the cat might be plugged up?

 

if the cat is glowing....nothing is getting threw it. your cat is clogged that poping is your exhuast trying to find a way out and breaking the exhuast pipe. or you didnt tighten your manifold down. but if your Cats are glowing then yes its cloged.

 

I changed both cats and no luck. One of the studs was broke off and had to be drilled/tapped up to a 10mm so I thought maybe the head was messed up in that process. Took off the head today and everything looks good. Still runs like crap and makes small popping noises, cat turns red, and barley goes 25-30. Tried different plugs as well as a last resort.