Chevrolet Repair: at start up, dirty fuel, dark smoke

Question
small amount of dark smoke 4.3 v 6 1992 s 10 pick up 91,000 mi.good oil preasure ,no fouled plugs ,runs well, mpg 20 +.is it time to rebuild  heads, replace motor,or buy new truck.    thank you for your time.

Answer
Hi Heim,
Is the 20 + MPG down from previous??
If you want to have the valve guide seals replaced, it can be done without removing the heads.
The dark smoke is possibly from dirty fuel injectors, so I would run some injector cleaner through it and let it go.
Van