Author Topic: 2.0 TFSI (AXX) - boost lag followed by overshoot  (Read 57 times)

Offline brian_badonde

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2.0 TFSI (AXX) - boost lag followed by overshoot
« on: October 09, 2019, 09:59:25 AM »

Got an issue I would appreciate some help diagnosing if you'd be so kind. First some background: it's an 05 GTI with an AXX engine with 113,000ish miles on the clock. Had a stage 1 remap at around 100,000 miles and a milltek turbo-back exhaust installed around the same time after the stock downpipe rotted through with rust and failed an MOT. Everything was fantastic until a few months ago - I started noticing an increasing lack of throttle response over the course of this summer. A scan revealed a buggered intake manifold runner actuator which I replaced which has not helped the situation really.

I went and did some logs and found that the boost is incredibly slow to build with revs, but when it does meet requested it overshoots somewhat, at which point the n75 does its thing and cracks open the wastegate. Attached a screenshot of a 4th gear pull from ~1700 rpm (note: not to redline as I value having a driving license but there's enough to see the issue).

The final symptom worth mentioning is there's a high pitched whine that I can hear whenever I'm on throttle and above around 2K rpm. It's difficult to describe but it sounds like it could be air escaping. It's definitely not the diverter valve as I'm using a GFB DV+ and boost is clearly holding at requested. For the same reason I can't see it being a boost leak. This leaves the n75 valve itself or, in my opinion more likely, the vacuum hoses connected to it. Or my wastegate actuator is buggered...I'd rather rule out the first two first though as they're much less expensive to replace!

Are either of these known weak points before I go replacing them? Is there a quick way to diagnose it? Any help would be much appreciated.