So I've been having issues with my 1989 S5 N/A GXL. It started when I had a leak in my lower radiator hose and I replaced it and decided to swap out my old but working parts with newer ( what I thought were better condition parts) like my throttle body, mass airflow, OMP, TPS, injectors, and a few unrelated body components.

The car afterwards would start and Idle perfectly but as soon as I touched the throttle it would die, odd. So I swap back my MAF and the issue persists, I jumped the diag port and ended up getting codes 10, 12,18, and 20.

I took it down to the lower intake manifold and started from there working my way back putting it all back to the way it was originally and added a new chassis ground strap.

Car started and did its 3000 rpm warmup and then ran rough at idle at like 700 dropping off between 500 and 700.

It then died and now when I start the car it always instantly catches and does a tiny rev but just dies.

I re timed the Crank Angle Sensor and swapped between 3 MAF sensors but made no difference. All engine codes were gone as of the point I stopped working on it.

I am ready to admit defeat and ask for people smarter than me to point out what I foolishly missed. I was very careful to ensure my wires and vacuum lines were connected securely and in their proper place. I tried a different ECU and have the same issue. Admittedly I tackled it with more of a shotgun blast of part swapping but my electrical gremlin hinting skills are duller than a bitter knife. Any suggestions will be warmly welcomed and I will fill in with whatever info I may have neglected if needed.