What do you use to clean the cooling system? The service manual for Yanmar 3GM30F tells to clean the cooling system before changing the fresh water. I use 50/50 dilution of the antifreezer. Would any one suggest what I should use to clean the system? I have not changed the antifreezer for several years. The manual states that the cooling water should be changed every 500 hours. I have not used the engine for 500 hours yet, but I think the cooland deteriorates as time goes by. Thanks for any suggestion.