Ok i Have an 87TII that i put a S5 J-Spec Motor into about two months ago. I swapped the front housing and Mech OMP, injectors and fuel rails, intake manifold, using stock S4 ECU and harness. Well ever since i put the motor in ive had a problem keeping it running. It starts and tries to run and will stay running if i give it LIGHT throttle. If i try to lay into it, it backfires, and is very hesitant to accelerate. After holding the throttle for a little bit it will start to idle at about 1k. Figure the no initial idle could be because the BACV valve is not plugged in, anyone verify this? I have tried several things such as AFM, TPS, all vacuum lines have been replaced, tried another ECU, fuel pump, fuel filter, plugs. All suggestions would help me greatly.

 

Misrouted vacuum lines or vacuum leak would be my first guess.

Is your bac valve not plugged in for some reason?