Added a bottle of diesel kleen to prevent smoking on 03 duramax, shortly after truck went into limp mode, i have changed fuel filter twice and it clears for about 2 minutes and then goes back into limp mode.Any info would be greatly appreciated.

When was it last given a Major service? Black smoke is usually a sign you engine is running 'Rich'. This means the Fuel and Air mixture has too much fuel in it. Are there any warning lights on the dashboard? If it is going into 'Limp Mode' the ECU, (the cars brain), has detected something it thinks is serious. You should get the vehicle plugged into a diagnostic system or an OBD2 scanner for the Error Codes as these are clues to the problem. Try a few different garages as the costs for this service can be very different.
Old Fuel and Air filters can alter the fuel mix. Other areas to check are the EGR valve, fuel injectors and the Catalytic converter.

It's not in limp mode , you put diesel in it . You need to pull the fuel tank out ,drain the diesel out , clean it out , clean fuel line's ,replace fuel filter etc..

Does the transmission have an over heat switch? That is the only thing I can think of. If so, you need to add another transmission cooler to assist the one you have for those heavy loads.

Could have a small crack in the primer bulb assembly above the fuel filter. Mine did the same thing. I added a small fuel pump from autozone to send more fuel under pressure. Nothing crazy. Maybe 7psi. Never had a problem again. Or just try a new primer bulb assembly. I think they even sell rebuild kits that come with new o rings also.

Check your fuel filter they have water seperators built in and a drain petcock on the bottom. Put a bucket or something to catch fuel underneath and open the drain to allow the filter to empty. I'd recommend replacing the filter between 90 days and 6 months and try adding some diesel kleen additive this works exceptionally well in the winter months. As long as the cylinders aren't misfirng you may have some restricted injectores. I have a fleet of diesel trucks and that is 90% of my problem with them. Especially if you use an auxillary or day tank to fill up from as moisture can seep in through the vent cap

Sounds like one of your injectors are failing, the same thing hapened to me, the symptoms of injector failure are shown as fuel leak, its worth letting an injector specialist check it.

duramax diesel trucks do not have a lift pump. start with the simple things make sure your fuel filter is tight and check the shreader vlave on the fuel rail if that don't help check the filter housing for a leaks arround the heater element or a crack in the housing itself

When were the fuel filters changed.sounds like you have water in the fuel.Drain the tank and try new fuel after changing fuel filter.If it was in limp mode you would not get the revs droping down then smoke then picking up.Limp mode only alows you to do about 40mph.

try to remove filter and pump out fuel from the tank. the gunk you loosened with "mechanic in a bottle" goes to filter then when ENGINE IS SHUT OFF GUNK SETTLES BACK IN LINE. NEED TO PUMP IT OUT TO SOLVE PROBLEM

mo me it sounds like low fuel volume i would check my lift pump