I was driving my C32 a month or so ago and at a stop light an old model s4 came reving up. As the light turned green I stomped it and it took off extremely slow and shifted gears hard and kept doing this. I took it to the dealer and they said it was the purge valve and replaced it. Then I drove it there was still a lose in horsepower so I took it back to the dealer and they said everything was in good shape and it should run fine. It still has a definite lose on horsepower, anyone know what it could be?

 

Probably a bad MAF. They start to fail but do not trip any warnings so the dealership can't pick-up the problem on the Mercedes diagnostic system.

 

put it on a dynometer to prove it. you should have anywhere from 270-300rwhp If it comes in a lot less than that...tell them to fix it or return your car under the lemon law