I have a 2003 stock except for fass fuel pump. I started with an intermittent lack of power then would run fine for days. Now it runs smooth but lacks power. Starts fine, some black exhaust smoke. seems to run best when cold. I checked the fca, filters, cleaned the tank screen , wiring all good. Did get a p0251 code. cp3?

 

Maybe cp3.

The first two troubleshooting tips are fuel pressure gauge (although I sincerely doubt it is that) and rail pressure gauge. Drive it and check the gauges versus what the truck is doing. Lack of power, smoke and elevated egts are an indication of low rail pressure. Normally this is seen when you have a programmer (or other mods, like big injectors)that exceeds the limits of the fuel supply.