They further diagnosed the problem with this as probably being the Turbo.
It is a good idea to see the limit of the travel of the VNT actuator before dismantling, and to see how smoothly it runs. This is easily achieved using a piece of rubber tubing form the car and pulling your own vacuum with your mouth. You will then see the limits of the travel.
The first part to remove is the vacuum actuator. This is held on with with 2 of the six 10mm bolts and is connected to the VNT mechanism with a circlip. Use the long nose pliers to remove the circlip. Once this is removed you can lay it to one side as there is nothing you can do with this part.
I took the precaution of marking up the outside of the turbo to make for easy alignment on re-assembly. This subsequently proved to be unnecessary as it can only go back together in one place.
Remove the remaining 4 10mm bolts, then with a soft hammer gently tap until the outer shell separates from the main body. Be sure to support the unit in such a way as the final separation does not cause contact with the turbine. Any damage to this will render the turbo unusable!
Once you have this part off you can also set the rest of the turbo aside until final cleaning and re-assembly. When cleaning this be very careful not to damage the turbine. On my turbo there was about .5mm of hardened soot. You can just make out at 4 o'clock in this photo the lever that connects the outside lever to the inside actuator ring.
You can clearly see a light dusting of very fine soot covering the outside of the VNT mechanism. At the 1 o'clock position you can see a space for the external actuator arm to locate into. At approx. 12, 4, and 8 o'clock there are 3 small rollers that guide the external ring locating and controlling the fingers. Very carefully lift these and the ring clear of the rest of the body.
With the ring and rollers removed. (Below)
Next you need to remove the 3 T20 torx screws located at about 10, 2 and 6 o'clock. These on my unit were very tight. I would not recommend using a cheap torx bit for this job. Once these are out you can lift the rest of the internal part of the VNT mechanism out.
This unit is sat on 3 small spacers.
Once you have got this far it is simply a case of cleaning all the components. I did this with a soft wire brush, scraper and wet & dry. Work carefully as the parts could damage easily.
Below are the after cleaning photos.
This project should not be outside the realm of anyone comfortable using general hand tools & common sense. Just work clean, and work carefully. The first one I did (this) one took me about 2 hours, the next one will take considerably less. I was working blind, at least you have this to refer too. Enjoy :-)
Once the internal VNT mechanism is clean and back together this is what it looks like.
You can now check to see how much smoother and fully the VNT mechanism moves.
The good news for me is my Allroad is now a lot, lot better with this turbo fitted. It is still not delivering the power I still expect or want, but I am beginning to think this is about normal for this car. On the TDiClub forum there is a FAQ that bears this out.
I would appreciate any feedback on this article, and welcome any suggestions on how to further improve on the sub 2200rpm power output.
I have a video file of the VNT moving HERE. It is quite a big file (16Mb) and will take a short while to download. When the ring is to the left it is at the low RPM, to the right it is once the n75 has released the vacuum. When it is to the left the bypass ports are shut forcing more exhaust over the turbine. When it is is open much of the exhaust is vented reducing the force on the turbine.
If you are suffering the same problem and this fixes it for you please let me know to firstname.lastname@example.org