I have a 2000 f 550 with the 7.3, when stating it up when its cold the engine runs rough and will not speed up, when it does it will smoke at higher RPM's, I redplaced the fuel filter, added fuel conditioner, replaced the wiring under the valve cover and glow plugs, sometimes after running the truck it seems to be fine .HELP

. Black smoke is usually a signal that there's too much fuel, not enough air, or injector pump timing is off. One of the most common causes is an air inlet restriction. The cause may be a dirty air filter, a collapsed intake hose, or an exhaust restriction.

. Check air filter--ensure it has been changed recently.

. Check fuel injectors. Leaking injectors create an overly rich air/fuel mixture and cause black smoke. Take the valve cover off and look at the exhaust part of the injector, when it is running. Each time the injector fires, you should see oil exhausting from it. If you see oil coming out, then the injector is good. When replacing a bad injector, take the glow plug out and crank the engine to get any oil out of the cylinder. If you don't, you could hydro-lock and bend a rod.

Thanks, but I didn't menton that the smoke is white not black at 2500 and above.

White smoke may be coolant being burned. Have system pressure checked for cracked head or gasket problem.