Erratic Idle after header replaced


Jan 1, 2017
Hi guys, I'm having an issue that I cant seem to remedy after replacing my drivers side header. The story started last week when my check engine light came on and I went to get the codes scanned, I cant remember the code but it had to do with my driver side bank running too rich, funny enough once I got home my engine sounded like a lawn mower and I immediately suspected an exhaust leak. After checking I had found that the collar of the header that attaches to the mid pipe had cracked clean off, at this point I'm assuming a hit a bad speed bump bending the exhaust and slowly causing it to crack.

So I order a new header pull out the old one and replace it with the new one, here is where I screw up royally and I'm still kicking myself, when I put the header in I removed the spark plug cables and when I went to put them back I screwed up the order like an idiot and scratched my head for all of 5 min when I was getting nasty backfires, to make matters worse I noticed my error and rewired the spark plugs only the screw up a second time (I know right how the hell does someone screw up placing 3 wires into their proper holes twice).

Finally I get the order right and the backfires are gone, however at this point my cat is glowing red from all the fuel being dumped from the firing order being off, so I give it a few hours to all cool down before I come back. So now I'm faced with idle being completely erratic, while in park it is only slightly rough but rough enough to notice, when the car is put into drive the rpms immediately plummet to a near stall but recovers, this plummet occurs anytime the break is pressed when the car is in gear. If the car is in park and I turn on the AC the rpms will begin to dance up and down like a yo-yo repeatedly, never stabilizing.

I'm thinking that there is a problem either with air or fuel ratio at this point, Ive pulled off the throttle body, idle air control valve, and mass airflow sensor cleaning all parts scrupulously, Ive done several checks to make sure no hoses are lose or wires unplugged. At this point I'm stumped on what to do next, I suppose maybe the IAC valve or MAF sensor could have gone bad, but I'm sorta lenient on playing the game of buying new parts until the issue gets found. The thing that really puzzles me though is that this issue occurred right after replacing the header, unless of course my screw-ups with the firing order and backfires managed to damage something else.

My mustang is the 04 V6 Base Model.
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