At a loss here, coming to ask for ideas on how I can diag my 2001 audi tt 225 (AMU engine code). Bought it very cheap a couple weeks ago to tinker with and have gone through the basics. Fluids checked, oil changed, car cleaned up. When I got it, the car was in soft limp mode, only boosting to roughly 4-5psi. Looking at MAF readings, it was doing roughly 110g/sec max airflow in 3rd at 5000 rpm. When I picked it up, it had codes for the b1s2 o2 sensor low resistance and voltage as well as system lean. I took off the Intake Manifold and fixed a couple of vacuum leaks in the mess under there, as well as replaced the N75 with an N75 J valve, and the rear o2 sensor with Bosch replacement.

When I finished those fixes, all the check engine lights went away. However, the car started sputtering and bucking really bad on acceleration between 3000-4000 rpm. If I keep the throttle between 0-20%, it drives pretty normal. If I go above 20%, it will sputter for about 10 seconds, then finally accelerate like it should. Idle is stable. If I unplug the MAF sensor, the car runs great. No misfires and pulls clean through the rev band. Of course, it only boosts to wastegate pressure, but it is very smooth. It does feel like it has more power than when I first got the car.

Are there any other possibilities? Could a bad n249 or DV (currently a Forge piston style which tested fine, but was a bit sticky) cause this? Could any other sensors getting slightly abnormal readings throw the car into a limp mode where the N249 opens the dv to modulate boost? Do I have any other ways to test other systems before just replacing the MAF?

Thanks in advance. I have VCDS lite and can use that to help, but don't have full version at this point in time.