There are two main cooling passage contaminants, and they respond to different treatments. I've never used CLR in a cooling system, but it's a good idea. I used technical grade Hydrochloric acid (muriatic) in the really rusty ones, 3 or 4 ounces of acid to a full fill of water and the thermostat removed. (no antifreeze) Run the engine with about half the radiator covered with cardboard to raise the operating temp. (Don't let it boil). Follow up with a fresh fill of water and a box of baking soda to neutralize the acid. Flush thoroughly and refill with a 50/50 water antifreeze mix.
For greasy crud, I use a can of drain-cleaning lye and water, plus 1/2 a cup of Tide. Circulate until hot, flush, and refill with water/antifreeze. Most commercial "radiator flush" products are far too weak to do any good. If the freeze plugs aren't new, be aware you might uncover some weak spots and cause seep leaks with this process.
Jerry
