Hi,these devices usually lie to the ECU about fuel delivery.I have never seen one or used one,but descriptions that I have read describe some as nothing more than a resistor in a box.
They strive to make the mixture richer,costing you extra fuel,and associated exhaust smoke,but apparently can be effective.
General consensus is that the Nitro CRD is already finely tuned, and that exhaust modifications can lead to the engine malfunction lamp and limp mode.Something to do with exhaust pressure being out of range?
Some owners of other diesel vehicles find these power boxes work well,others don't.