Hello I have a 99 dakota slt with a 3.9 and I swapped engines about 3 months ago and everything was good but now its missing at idle but no codes what so ever. I've changed plugs, wires, checked compression and fuel pressure, which I replaced the fuel due to pressure at 44-45 now its 50-55 but no change in my issue. Rough idle and sometimes when I go to accelerate it misses and backfires. Any clue? Please help

What plugs did you put in it? Did you change the cap and rotor as well?

I put the OE champion plugs in it and no I haven't changed cap and rotor yet. I will tomorrow tho