I have a 95 chevy 4.3 vortec vin w with cpi had it scanned it came up misfire in bank 5 and o2 sensor replaced o2 and ran injector cleaner in it and it started blowing out black smoke took out all 6plugs and all of them where covered with black carbon does anyone have any ideal what could be causing this.

 

Test your fuel pressure paying particular attention to how the pressure holds after the pump shuts off.