Black soot also means short trips in the winter time.. The cold engine is requiring a richer mixture until it warms up. Your computer knows the outside temp and the coolant temp and leans the mixture as much as it can. Make a tank trip and see what the milage is.. Also as summer comes around see if the milage increases..
As a general rule if the computerized engine is running smoothly, without lights showing on the dash, you are getting the best it can give.
Oh yeah, about the air cleaner, I think there is even a message for that.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />