as many of you know i have removed my mass air meter and use a programmable fuel computer for my fuel curve. Now I have the thing tuned in pretty damn well but about 2 weeks ago i started getting dark soot in my tail pipe...I looked at my fuel program as well as doing diagnostics w vag tool, yeah the car was running richer(data block 1) than normal by 5-7% but no check engine lights and also no DTC's, turns out it was the coolant temp sensor, audi has superseded this sensor with a new part #. Only $5 and 5 minutes and the car was back to being perfect! Guys although the damn mass air meter sucks A$$(in 2000 S4's)the coolant temp sensor can also cause fueling fluctuations without giving you much of a heads up! these rich conditions can lead to cat as well as O2 sensor fouling and failure and can put the car in failsafe mode further heightening the already rich conditions!

***things to look for***
1) erratic coolant temps on your gauge
2) high or low coolant temps on vag tool (data block 1) especially on cold start up(I believe the default value is 189 degrees for the sensor but this will not show up on your instrument cluster gauge)
3)too high or too low fuel trim(data block 1) I believe anything +/- 10% under idle and part throttle is a red flag!(if you have this and its not the coolant temp sensor you can also look at mass air levels at idle(i believe 4.8-6.3 g/sec is a good range**its been a while since i had a mass air**(data block 2) and also make sure your primary O2 sensors are trying to maintain stoichiometry(sorry i dont remember the data block)