nixer

First of all I know... this topic has been discussed many times here and in AZ forums. I have spent days trying to understand what is wrong with my car and why this code.

No CEL. The car is slow and sluggish .. plus very bad mpg... around 15mpg (city+hwy) even with conservative driving.

I learnt that positive deviation - overboosting and negative deviation - underboosting .. I could be wrong though.

I did a pressure test and couldn't find any leak.
Before ordering N75 and DVs, I wanted to do some data logging...I basically logged N75 duty cycle, and block 115. I see definite boost loss. My question is... how does the N75 duty cycles look? How about MAF readings? What do you guys think could be the problem. I'd like to know your opinion before spending money on the N75 and 710Ns.

The N75 duty cycle is always at 10.2%... doesn't look right to me!



Thanks in advance!!

nixer

Anyone?
Is the ECU requesting more boost than it should? My N75 duty cycle % is always 10.2%.

NefMoto

Have you scanned for codes with VAGCOM? You can have codes stored and not have a CEL.

Have you checked your long term fuel trims?

Your actual boost is lower than your requested boost, which would be a negative deviation. I would do another log showing load and requested load.

nixer

The only code I have is the positive deviation!

nixer

Anyone? Is the requested boost around 11 psi normal for a stock car?

jamesdmanley

its probably just an old code. clear and and see what comes back. looks like youre running in limp mode

Peter Badore

I think 11 psi is normal for a stock US market S4, but there isn't a normal boost pressure curve in the Audi 2.7l biturbo self study program book only a description of the limp in mode called "basic charge pressure" by Audi. If the wastegate bypass regulator valve N75 fails according to the self study guide the charge pressure is restricted to 4.4 to 5.8 psi (300 to 400 mbar) which looks like your situation. The self study guide identifies a number of sensor failures that can restrict boost pressure including the exhaust temperature sensors (G235 and G236), the charge air pressure sensor (G31), and the barometer pressure sensor. Unfortunately not much detail is provided as to operation of the engine in limp in mode. I was surprised to see any activity in your N75 valve; I would have thought the valve would have stayed shut at 0% based on the self study description of "basic charge pressure". Maybe different failures have different boost pressure parameters.