Hi everyone. Im new to this forum so I will present myself. I am Jean-Philippe Gauthier from Québec, Canada... studying mechanical engineering at Sherbrooke University. My mother owns a 2000 S4 since new, it is now 156000kms.

Here is the problem: Since new, the car always impressed me for its acceleration. But now, it seems that from 3500rpm to the red line, power just turns off some times. Feels like a 2.8 a4...

What I've replaced:
DVs
TBB
Air filter
spark plugs

I was thinking of changing the fuel filter and cleaning the MAF tomorrow.

I noticed today that the engine didnt make its caracteristic 'air rushing through a pipe' noise when the problem appears. Some times, the car runs just like new and this sound is clearly audible. But some time after, no more power, no more sound...

Besides that random underpower problem, the car runs perfect. No smoke, starts fine, idles fine, super smooth all around

Does someone have an idea about what should I check next?? Does the 75N valve could be the problem?

thank you very much

 

have you checked for fault codes? it could be the fuel filter if you havnt changed it or the ECU needs to be reset or upgraded to handle the modifications,
VAG it and do some data loging

 

Check for stored codes....

Then unplug the MAF & see how it runs, if it's the same, then the MAF is fine, if it's better, then replace the MAF.

Next on the list would be the N75 valve. Mine was going out & giving the same symptoms, no codes.

Lastly, just change out the coolant temp sensor, it's cheap, & easy.