On the really badly gunked-up ones when I worked at a sand, gravel, and concrete business, I've actually used the same Muratic Acid we used to clean bricks and other masonry jobs. Dump 3-4 ounces in the radiator, top off with water, and run it until the engine temp comes up. Drain, flush with clear water, and dump a couple of boxes of baking soda into the final rinse to neutralize the remaining acid. For grease/oil contamination, flush with a good dose of powdered Trisodium Phosphate added to the water (concrete cleaner). All these chemicals are used in very weak solution in most of the commercial cooling system cleaners- - - -just use the real stuff in a very heavy dose to clean out the bad contaminants!
Jerry
