Due to cavitation, it's extremely important to take proper care of a diesel engine's cooling system. Use only a 50/50 mixture of recommended antifreeze and distilled water. Be sure and add the necessary supplemental cooling additive (SCA), when changing coolant, and check/add additional SCA every 6 months. Change coolant every 2 to 3 years, depending on use. Check for possible blockage in the radiator, due to corrosion or mineral buildup. You may need to have the radiator rodded or replaced.
I've never used this product, but you might want to give it a try:
Super Cleaner