'99 2.5 TDi Boost issue.

[jezzo]

First post and I'll be as detailed as possible here...

There appears to be a boost issue with the old girl (A6 C5 Avant Quat). I was forever getting a flat spot around 2700rpm in 3rd, 3000rpm in 4th and then in 5th no acceleration past 3000. If i ever hit kick down it would just rev hard and instantly slow down. very dangerous when overtaking on motorways or anywhere when there's a car up the rear... No black smoke though. I have checked nearly everything, scouring forums and garages for answers but to no avail. I checked vacuum lines to turbo actuator, no apparent issues, N75 seems good, the turbo spools up nicely and the actuator is moving freely. Had a new MAF last year but not sure if this could fail again so quickly - I do do a fair few miles, long journeys.

As advised by one person, I bypassed the EGR last weekend and all of a sudden it felt like I had power all the way up the band - smooth acceleration, perfect, a real noticeable improvement and I was very happy at this point... until I heard a new hissing noise coming from what sounded like behind the dash. I popped the bonnet and got my mate to rev it a little. When the turbo spooled up, a loud hiss was generated. I tracked it down to the small 3 inch wide rubber pressure hose connecting the turbo to the top intercooler pipe - there was a small hole worn through underneath it and so I dashed of to the local Audi parts place and bought a nearly new one for a fiver. Got back, fitted it and I thought that would fix it but no... I Took her for a spin and low and behold, boost issue was back. Before the hole was fixed I am assuming it must have been the hole in the connecting intercooler hose was letting pressure out and stopping overboost - perhaps before the pressure got to the throttle body or perhaps a sensor - That I am not sure about! Does the throttle body actuator run from a vacuum hose and is that prone to splitting? Could that be sticking somehow. Would the boost pressure sensor be causing this? Or is it simply a case of another bad MAF? No way to test it on this by disconnecting as it just goes into limp mode.

It is really bugging me as when the was a hole there it drove perfectly. The turbo would spin up and whistle happily, the gear changes were smooth. now back to square one. I know that usually if there's a hole in the intercooler pipe, there can be these symptoms but I have not found a case where a hole in the pipe has actually made the car run better.

I guess what I am asking now is, Has anyone experienced this and know how to fix this. I am going nuts!! I know it needs to go on a VAGCOM but before the garage starts telling me I need to change this and that, I would prefer to try all avenues myself.

[kite]

Look for posts by adamss24 (Chris) you should find his number. He is the man for 2.5 V6 and he is in London..

[ametlib]

Hi,
I'll say it's 95 % chance the vanes in your turbo is clogged up. Let the engine idle and take off the vaacum line to the VNT clock and then put it back again. The VNT arm should mowe smoothly from the stop screw and out then back again.
If not so, you need to clean the turbo or get a new one

[PaulF2.5TDi]

+1 to Ametlib.

Google 'Mr Muscle Turbo Clean' for various 'how-tos'

[jezzo]

Thanks. Read up on this and will give Mr Muscle a try and will let you know how it goes but for now I have another problem and can't even move.