TT 225 Sputtering, No CEL

[soysauce234]

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.

[Seal1968]

If it runs better with the MAF disconnected, that might indicate that it is either faulty or dirty. The boost control valve right behind the MAF could have a vacuum leak too, so check that. N249 helps control overboost I believe, so if you unplug it it will throw a CEL. I tried to delete with one of those plugs from Uro Tuning and run the vacuum line directly from the manifold to the diverter valve but it didn't work right, so I sourced a new one and ran all new lines which fixed that issue.

Unless your running a tune a stock updated diverter valve is pretty cheap as well.

Dealing with a cylinder 4 failure myself with my AMU 225...hole in the piston and scratched walls...going to swap out another good used block and transfer all my goodies over.

Sorry to say that the MK1 forum is basically dead...hope I helped.

[soysauce234]

Thanks for the input. Most forums are, unfortunately, pretty dead now. Still chasing the problem. I have more money than time so I'm just going to pretty much replace the entire vacuum system and Maf sensor. I did a few tests blocking off the DV, bypassing n249 and nothing changed so I know those are fine. Swapped back to the old N75 with no change. It's not a major boost leak, I did a boost pressure test and no noticeable leaks. Sprayed carb cleaner on every single vacuum line and I cannot find a leak.

At this point, I am sure it has to either be the PCV/PRV or the MAF sensor. I've ordered all of those and will install one at a time to see what works. Because those are the only things I can think of.