Overboosting w/ RS6 turbos ... can't figure it out. Searched, no dice

B5 S4/ RS4
#1

So i recently bought this car and talking to the previous owner he was telling me the car was tuned for 22 psi tapering to 19 towards redline. This is on a VAST tune. Seeing how i tried to different tunes and they both do the same i am tending to believe i have a hardware issues vs a software issue. Description below.

My troubleshooting:

Round 1.

I am peaking 26-27 psi according to the boost gauge and this thing seems pretty accurate showing 20 mm/Hg vacuum at idle and resetting to 0 with engine off. Also i can feel the boost coming in as the gauge starts indicating it from 0 and on.

No vaccum leaks, idle is rock solid, passed SMOG today. Scanned with VAG COM and no codes are present. Checked for an MBC and none is present. Gauge is a 30psi JHM boost gauge. N75 is said to be new(can i check it?). Boost builds gradually and stays solid until you take your foot out of it.

Round 2.

Swapped files to a different tune made for Franken turbos. This is also a 22 psi file, that i was able to borrow from a local member for SMOG purposes.

Same thing here … this one spikes to 28-29 psi and would probably keep going if i were to keep my foot in it. I even blew off an intercooler hose today.

No vaccum leaks that i can detect, idle is rock solid, passed SMOG today. Scanned with VAG COM and no codes are present. Checked for an MBC and none is present. Gauge is the same as above. No other changes to the car.

Question:

so for all you gurus … what could my issue be? i am kind of scared to drive it this way because i might be pushing the stock block too far

i have a vagcom so i am all ears as to how and what i should check to find the root cause for this.

Also i would rather not have to add an MBC in parallel.(most of the other threads on the subject suggest this but im not too keen on it)

thanks in advance for all your valuable suggestions.

#2

log block 118 to see n75 duty.
what n75 is on there, i believe the correct one is the n75j… my memory isn’t the best though, might be n75b.

the wrong n75 can cause an overboost issue.

did you specifically check your turbo reference lines for leaks? I would do that as well. While you are at it check the cracking pressure on your WG’s.

#3

  1. what should i see for n75 values? 100% at full boost? not sure what to look for here … maybe you can help out and give me a clue. i am a n00b when it comes to this vag com/audi stuff. ll check the part number and get back to you.

  2. the turbo reference lines go from where to where? would these need to be checked on a lift or do i have access from the top? how do i check them?i am mechanically inclined but again dont know much about this specific setup. how do i check the opening pressure on the WG?

i appreciate your help so far and apologize for not knowing more :slight_smile: i’m a fresh transfer from the BMW world.

#4

Ok so the n75 is the solenoid that controls boost by bleeding air away from the wg actuator.

there is a ambient reference line that comes off the spider hose.

then there is the turbo reference line that comes off the compressor housing

There is the WG reference line which lets pressure get to the wg actuator

you can check cracking pressure and pressure test the wg reference lines at the same time. find where the lines T off the n75 and unplug them there so you can check each turbo individually, or do both at the same time if you like.

using a compressor with a regulator run pressure to these lines. you will hear the wg actuate as long as the compressor isn’t running. gradually increase pressure till you hear the wg actuate.

on stock cracking pressure you will see about 85% dc for 22-24 psi generally speaking.

#5

your obviously in good hands here with judeisnotobscure. If your looking for a quick fix and also something that will help with the testing. Toss in a MBC parallel or standalone. From there you can see what the waste gate pressure is set at. This will help give you more control of the boost to see if its hardware or software related.

If you put in the MBC and you still are getting no control of the boost then you have a preload issue on teh WG.