870 loss of power and bad fuel mileage

[DVT873]

Did this come on gradually or all at once? Black smoke is out of balence air fuel mix. Coming on gradually to me would point to injectors wearing out and ovefueling. In the medium stuff I deal with anything past 150K miles or 5000 hrs are suspect. Rawze probably knows the numbers for ISX's. If it came on sudden it's not getting enough good air. Does it have a working EGR valve? (I'm guessing it doesn't). I see a lot of those that don't fully close when they should but don't set a code. Some on ISB's, alot on NAV medium. Puts "dead" air in the mix.

Maybe something wrong with the VGT? They usually code but not always. Vane travel might be off. Get a manual gauge on it to confirm what the ECM is seeing. Is your incab boost gauge manual or electronic?