Wednesday, 31 January 2007

De-Cokeing a VNT Turbo

I have been having a few issues with my 2002 Audi Allroad 2.5 TDi V6.The tails of woe are over at the Audi Sport Forum do a search for Biggles.

Anyway after trying many of the suggestions on there still no significant improvement was noticed, so I bit the bullet and took it to the local Audi dealership who diagnosed and fixed a problem with the diesel pump timing. It was 2 teeth out on the belt although the crank & cam timing was OK. The results were very much better, but not solved. The problem of low power at under 2200rpm was still there, although better, but over 2200rpm there was plenty of power.

They further diagnosed the problem with this as probably being the Turbo.

The type fitted to this car is of the VNT type.

Audi wanted about £2000 to fit a new turbo at £1700 for the Turbo + fitting + Vat. I said I would think about it, and decided to do some price comparisons on the net. It is pretty easy to source a new one for around £500.00 (I am told at the moment this is not supported by Garrett with spares for re-manufacture, an only new units are available). Of course this my well change by the time you read this article.

During my search I came across a known problem with the Turbo, that is one of sticking VNT mechanism.

On chatting with another member of the AudiSport forum that had a similar problem solved by the fitting of a new turbo, he offered me his old removed turbo to dismantle. My thanks to him for the provision of this.

Below is what happened next:-

Note:- Tools needed. Long nose pliers, 10mm spanner, "soft" hammer, soft wire brush, fine wet & dry / emery paper paint scraper and T20 torx bit. I would recommend the wearing of rubber gloves as well, as the soot is very fine and stains your hands, and is a bugger to get clean again.

The turbo was thoroughly examined externally for any signs of damage or cracking. None was found. As the bearings were know to be good the only part of the unit serviced was the exhaust turbine side.

The oil feed and return ports were blocked with kitchen towel to prevent ingress of foreign objects

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.

The re-assembly is an exact reverse process. Make sure to align all the parts as they were before dismantling. Pay particular attention the to position of the fins on the inside make sure they all lap the same way as in the picture above. This is very important. When you replace the outer body back you need to line up the small roll pin with the hole at the 12 o'clock position in the picture below, then gently move the outer lever until it engages the inner ring.

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


Anonymous said...

Nice job and great photos. You may need to cane your Audi a bit more often to stop it coking up again!

Anonymous said...

Great tuition mate im just about to do mine by following your blog so ill let you know how it goes

Tin said...

I have exactly the same problem of very little power under 2200 rpm on my Allroad, but alas my technical prowess means I'm not confident rebuilding the turbo myself. I think I'll try to source a replacement, or print your instructions out for my local garage... if they're up for it.

Thanks so much for taking the time to help others out

Kev's Blog Space said...

Hello Tin

It will certainly be worth while trying it. By all means print it off.

This is not a rebuild though it is a simple de-coking of the VNT mechanism. You DO NOT go near the turbine as this is very delicate. Any soot on this should be left where it is. THe problem is not there.


Anonymous said...

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SA Begger said...

A. Archtect.

This site is great, thanks! I've hade the same problem with my TDI and after i've done the turbo my self for the first time, I came across this site, but TDI's running smooth. thanks once again.

Fim said...

Kev - I'm giving my car in for a de-coke - should I instead have the turbo reconditioned? The price difference is nearly £200. My car has 120k on the clock... what dya think?

Kev's Blog Space said...


Not sure really. What is the expected life of a turbo? How long do you plan on keeping the car?

I would think if all is going well save your money until it goes pop. It might never happen with luck.