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I use a Prestone Flushing Tee in the Heater inlet hose. I follow the directions on the Prestone Super Flush bottle . When it comes time to drain the system, i open up all engine block drains and radiator drain valve keeping the radiator cap on. I have a hose hooked up to the flushing tee and i force the water out of the block drains and radiator drain. To finish the procedure, i shut off the tap water and let the entire cooling system empty out. Then i take my compressed air hose and blow any remaining water out of the system. Then i refill the system with 100 percent distilled water with 2 bottles of water wetter. (Just prior to winter storage, i add a gallon of antifreeze since i keep it in a temperature controlled garage at 40 degrees F. ).
I do this cleaning procedure each Spring in addition to power washing the radiator fins and my oil cooler fins.I also do the entire outside of the engine. I know im an obsessive - compulsive Vette owner, but i still love myself.
our mechanic at work recomends using about a 1/2 box of dishwasher soap,drain the system, add some water,then soap,fill it up with water and run around town for about 25 minutes,getting it good and hot,then drain it before it cools. i did this on my 94 suburban,before putting new hoses and thermostat,and replacing the cast piece on the intake manifold,which was corroded and clogged,preventing heat in the interior. anyway it seemed to clean out everything,but it really wasn't that dirty to begin with. you could take it to a radiator shop and have them "boil" it out,but this sometimes promotes leaks,which the radiator shop will have to fix= more $$$$. back in the 50's(yes i'm that old) guys used to Coca-Cola to clean a radiator!!!! but that was the old formula Coke!!!! my radiator on my vette leaked so bad when i got it 5 yrs ago,i replaced it with a Modine brand radiator,about $265.