You probably need to know a little more about your car before decide what to do. Some cars have only drain plug at the radiator, which by itself can drain only part of the coolant in the system. Some cars have drain plug at the engine block too and would drain out more. However, there will still be old coolant, which could be as high as 20-25%, in the system even for cars have both. There are coolant flush machines that can suck out all of them. But for people who do it themself, the viable way is to fill the system will distilled water, start the engine till it gets to operating temperature, let it cool down, and repeat the whole drain process agian for a few times so that the old coolant is diluted to below some acceptible level. (My personal goal is less than 10%.) Since their is still water in the system, the new coolant must be the concentrated one to get the right mix. You fill the new coolant first but only half the capacity of the whole coolant system (user's manual should has the number) and then fill it to full level with distilled water. However, somehow I got the impression from reading past postings here that Honda only sells 50/50 diluted coolant. No concentrated. If that is true and you choose to stay with Honda coolant, take Al's suggestion, just drain the old one and fill with new one. Depending on how much you can drain out, you may have to do it a few more times over the life span of the coolant. If you choose to use a substitute, you have no choice but get as much old coolant out as possible since mixing two different coolants is not recommended.
[ October 01, 2005, 12:48 AM: Message edited by: rwl408 ]
