Audi a3 2.0 tdi 2007 underboost problem

[handjamjack]

I've a 2.0TDI which wasn't boosting properly and was going into limp mode and showing an underboost code. After fitting replacement MAF, MAP, EGR, actuator and boost pressure sensor the situation was no better. Found that taking the vacuum pipe off the vacuum reservoir restored very good boost pressure and the car ran as it should, though still went into limp mode after 30 miles and eventually came up with EML and underboost fault code. Anyone know why removing the vacuum pipe from the vacuum reservoir would result in normal boost being restored? Thank you.

[Renars]

Its because vacuum is leaking somewhere.

[handjamjack]

Its because vacuum is leaking somewhere.
Thanks for that - had a smoke test done with negative results

[handjamjack]

Still struggling with this one - have posed the question to my local mechanics and VW/Audi specialist plus 2 Audi master technicians and no-one can give an explanation. The issue is - A3 2006 2.0 TDI won't boost unless the vacuum pipe is removed from the vacuum reservoir. Without removing this vacuum pipe the car runs like a non-turbo........remove the pipe and it boosts normally. If anyone can shed any light on this I'd appreciate it very much. Thanks

[Renars]

Turbo is not moving vanes as its supposed to.
I feel its vacuum related by what you said before. Check everything vacuum related for turbo vane activation. Try to move actuator lever by hand does that move freely?

[handjamjack]

Thanks very much for that. Actuator has been tested and moves freely....actuator is new, maf and map replaced, egr replaced, N75 replaced. It's looking like the turbo, but would still love someone to answer the question as to why removing the pipe from the vacuum reservoir allows the turbo to boost normally.

[Renars]

Thanks very much for that. Actuator has been tested and moves freely....actuator is new, maf and map replaced, egr replaced, N75 replaced. It's looking like the turbo, but would still love someone to answer the question as to why removing the pipe from the vacuum reservoir allows the turbo to boost normally.

Thats what is confusing because if vanes are mechanically sticking removed pipe would still make it to lack boost.

[Crasher]

What is the fault code number? Has the vacuum system from the servo pipe been checked for integrity? Occasionally you will get an underboost code due to a clogged turbo but it is not common.

[handjamjack]

Hi and thanks very much for the reply. The fault code is 2099. Garage did a smoke test on the vacuum pipes with no result but I am not sure if this would have included the servo pipe end? I'm now trying to work out the relationship between removing the pipe from the vacuum reservoir (which allows the turbo to boost well, but with occasional limp mode and eventually code 2099) and any issue with the servo pipe. I just can't get my head round the fact that removing the vacuum pipe from the vacuum reservoir restores boost when it seems like it should be the opposite. Brain's hurting!

[Crasher]

P2099? The basic test is extraordinarily simple, remove the vac hose from the servo check valve and apply a vacuum (the Mk1 mouth is useful here) and you must not be able to draw air, if you cannot them it MAY not be a vacuum system leak BUT it still could be a post valve leak. I would like to see a proper VAG specific code read result and know that the actuator has been driven in basic settings (double checked in output tests) and the boost figure chnage observed and noted at raised idle.