2.0 TFSI (AXX) - boost lag followed by overshoot

[brian_badonde]

Folks,

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.

Cheers.

[brian_badonde]

In case anyone is interested and suffering similar issues, the problem stemmed from a knackered intake manifold runner control (IMRC) motor which operates the runner flaps. The IMRC unit operates by sending current to a motor which rotates a pivot attached to an arm - this arm attaches to the runner flap spindle in the intake manifold and adjusts the angle of the flaps as a function of, as far as I can tell, revs and/or engine load. The unit itself is only driven in one direction by the motor and relies on spring tension to return it to its original position. A spring is used so that the flaps open instantaneously when you bury the throttle to maximise airflow into the chambers.

However, as in my case, the main shaft bearing in the motor can fail and seize over time, adding friction to the system and adding delay to the flaps opening. This can cause some weird drivability issues and give you a flat spot in acceleration as I experienced. People all over tinterwebs will tell you that you have to buy a whole new unit at a cost of well over £100, when in actual fact the only component in the IMRC that failed was the motor. I cracked the unit open and it was in great condition bar a buggered motor.

Tracking down the motor wasn't easy as the company that manufactures them only operates B to B...but you can buy them individually from aliexpress for the eye-watering sum of...~£10: https://www.aliexpress.com/item/32980964393.html

NB: this particular motor will only work in early 2.0 TFSI engines i.e. AXX, BPY, BWA engine codes. It will fit the IMRC with part number 06F133482 - if your IMRC unit is printed with a part number with a letter at the end, you will probably have to find a different motor.

Happy to report that the new motor works flawlessly so far - its returned the throttle response and gives much better fuel economy when cruising. Removing the IMRC was far more of a pig than replacing the motor itself, if anyone'e interested I'll take some pictures and do a DIY.