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Hi I have a 2000 Honda Civic. There seems to be a lot of rust in the coolant. I just got it back from the mechanic who repaired a failed water pump. Any suggestions on what to do to fix this? Thanks.

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There are two methods I've heard of for removing rust from a cooling system. Both involve using a weak acid.

The first involves running a 50 / 50 solution of distilled water and vinegar in the engine for a day or two. There is a very good explanation of what he did here. I used this on my 98 Mazda 626 with moderate success.

The one you specifically asked about is the citrus flush, which is attributed to Mercedes Benz. You can even buy official Citrus Flush kits for various Mercedes Benz vehicles. There is a nice tutorial on this with lots of pictures here.

According to this site you can just use a 10% solution of distilled water and citric acid, something like 2.2 pounds of citric acid in five liters of water. You basically need to drain and flush all coolant from the system, then refill with the CA solution, run the engine for 15 minutes at 2500 rpm, drain and flush to remove all solution, then refill with coolant and check for leaks.

I must point out that there is some anecdotal evidence that there is a certain risk to this procedure:

Search under my name...Ive done the citrus flush twice and both times other components have been taken out, like the radiator, water pump etc. Im not saying dont do it, just be ready - anything that is on the edge of failure will be pushed over. Have your credit card warmed up and parts supplier ready forewarned :-)

In my opinion, if your cooling system is functioning properly, I would not do the citrus flush.

So do this at your own risk.

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  • I use a diluted muratic acid mix with water, then flush with water, then flush with baking soda and water mix, then flush again with water.
    – Moab
    Aug 28 '16 at 19:52

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