So, I'm still kinda new to this forum, so I hope it isn't bad form for me to start a new thread piggybacking on an existing one, but here's the deal..... I have been having an issue that, so far, no one has been able to identify...

I have a B5 1.8T with what I believe to be an APR Stage I+ ECU flash, a bunch of upgraded plumbing, and a Forge Diverter valve.

I Started developing this problem where, as soon as the boost pressure hits 10PSI (normally tops out at 21) something is dumping boost in a rapid fluttering sort of way, there is a buffeting or fluttering noise, the whole engine vibrates, massive power loss etc. The boost flutters very rapidly between 7-10PSI but cannot build higher than 10.

Everyone was convinced it was the N75 valve, which I replaced with no change in the issue. I disassembled, cleaned, regreased, tested and reinstalled the Forge DV valve. Checked all the vacuum hoses, checked all the EVAP hoses and connections, checked everything and I cannot determine what the issue is. It is very violent and upsetting when it occurs. I've been driving like an old lady to try and prevent the boost getting beyond 7PSI. Today I disconnected the Turbocharger Wastegate actuator valve from the N75 just to see what would happen... no change and problem remains.

What in the world could this possibly be? Car has consistently thrown P1476 Insufficient Vacuum EVAP since I've owned the car. The car was checked twice with the fogger for vacuum leaks and none were found. The tech determined the most likely culprit was a failed leak detector pump at the rear driver side wheel well. Since the was actually no leak and perfect vacuum, we didnt replace it... (it was a very expensive part). However, the car ran perfectly this way for nearly 10KMI before this new issue started, there still aren't any vacuum leaks so I don't see how the two could possibly be related...

Anyone have any idea what this could possibly be?

Could it possibly not be related to boost at all? I guess I could see how maybe a cylinder or injector could start misfiring... but at all different points in the rev range but always at 10PSI?

 

I would have instantly said something is wrong with the wastegate, until you said this...
can you elaborate on how you performed this task? dont mean to sound like a *****, but do you know exactly what your doing? again, im not saying your dumb, just asking if you know exactly which hoses to remove and how the n75 works. if youre doing all the work yourself, and again, know what your doing, hook the wastegate acuator directly up to the manifold (better yet, the chrage piping if you have a way to hook a nipple up). the k03 wastegate is only 3-4 psi i believe, and if you boost over that something is wrong with the wasteate. id like more information, whatver you have, share