Engine Insane fuel trims +25% LTFT Bank 1, -16% LTFT Bank 2

[imw_stl]

I have a 1996 GT and it's running extremely rough. When driving, my AFR is all over the place. It can sometimes be as low as 10:1 then as high as 19:1. I can occasionally hear backfiring while climbing to 3k-4k. Idle isn't smooth either, it'll almost look like a misfire but then smooth itself out. I pulled out a few of my brand new spark plugs and they were covered in carbon.

When I hook up my scanner my LTFT B1 is +25%, and LTFT B2 is -16%. My STFTs both are all over the place while driving, sometimes they'll hit 0% at idle but it's rare. Everything else looked normal, no codes set. I just replaced intake manifold gaskets and couldn't find any vacuum leaks. I'm pulling my hair out trying to figure this out, so any help is appreciated.

Engine mods : PI Headswap, Comp Cam XE262AH, 24 lb/hr fuel injectors, Edelbrock Victor Jr intake manifold & fuel rails, BBK 75mm TB & Plenum, BBK Longtube headers, Evil Energy Adjustable FPR

 

[PSKSAM2]

Did it start doing this after you did all those mods or just "all of a sudden"?

Any chance the sensor cables got swapped banks on reinstall? Honestly not sure this is possible, but sounds like one side can't adjust lean enough and the other not rich enough. I'm surprised no codes.

 

[Mustang5L5]

My stock GT had some pretty crazy fuel trims on one bank and smelled rich at times. No codes, chased a few things. In the end i decided to fire off the parts cannon anyway and blindly replace the two original upstream O2 sensors and that fixed the issues.