I have a 2000 A4 Quattro Avant. I am trying to find out how the N249 valve is controlled by the ECU. I am getting 17965/P1557/005463 - Charge Pressure Control: Positive Deviation. I have replaced the N249 (used), DV (710N-new) and the N75 valve (new). I don't have an overboost problem, the actual pressure matches the ECU on acceleration or full throttle, but when you part throttle or let off the throttle and the ECU calls for 960mb the turbo does not go down to the requested level. It then goes into limp mode. I suspect the wastegate and wg cylinder are working fine, I can see it move. I did a pressure test on the cylinder and it moves and holds pressure. No leakdown. Thanks, Nick

 

After replacing everything imaginable, except the turbo itself, what it came down to was the MAF sensor. It was giving a high reading and was causing the positive deviation. I bought it new from an ebay seller, so I'm not convinced it was the correct sensor for the car. I unplugged the MAF and ran it for a week. I got the CEL for the MAF, but did not get the positive deviation at all. Hmm. Long story short, I put a cable tie around the opening of the MAF, and haven't had any trouble at all. Finally! My log file showed 132 g/sec, WOT, and it's not complaining. I connected LED's to the N249 valve and the N75 valve. I can see exactly what each valve is doing and when. Just as an FYI.

 

good work! The MAF can be very troublesome

 

nice troubleshooting!

 

Thanks guys! I replaced the MAF and didn't look at it anymore once it took care of the fuel trim fault. Now I have a project BMW with a short term fuel trim fault. I might get good at this stuff!!
Thanks again!!!