[s-cars] 1993 S4 - need some help

[Evan Levine]

Long time reader.... not often poster, usually stick to AW. But I'm
having a long standing problem that no one seems to be able to help me
with... so I'm turning this one to the experts.

1993 UrS4 - Pastore chip, samco hoses, constant torque clamps - yada
yada other mods that don't really work into this equation. There is no
boost leak.

Car keeps running rich, I can see it on my A/F gauge and in my fuel
milage, also the black nasties that come out the pipe when I get on
it.

This was diagnosed for months by me to no solution, and finally my
mechanic tried cleaning some sensors, and magically it went away...
for about 2 weeks. Now it's back. Do I need to clean these sensors
better? Was it just luck and the weather or something is why I thought
it was fixed for 2 weeks? If the sensor needs constant "cleaning" is
that an indicator that it is bad and should just be replaced?

Any input on why it's running rich would be greatly appreciated. 

Evan