Hey Rufrob - I've seen the Mass Air flow Sensor cause some strange (Lean-Rich-Poor Idle-No Power-etc....) symptoms. So they can be difficult to troubleshoot without a known good spare. You were right to try and see what happens with the MAF disconnected. One of the MAF troubleshooting steps is to disconnect it to see if it runs better-worse-no change. The LH computer has a learning curve it goes through when the MAF sensor is reconnected and/or replaced.
Since your S4 doesn't use the feedback signal from an O2 sensor - the Air/Fuel ratio is calculated as a default open loop system. You might try to calibrate your MAF using it's potenimeter adjusting screw on the computer near the electric panel. You'll need an exhaust gas analyzer to try and set the potentiometer for a 1% CO at idle.
All the S4 models I've worked on had catalytic converters and O2 sensors so I appoligize that some of my trouble shooting suggestions are "By Guess and By Golly....
Good Luck, Michael
