Drain the entire system. Flush it with water first, so you see a clear return. Dilute the citric acid powder in about 4.5 liters of water. Dump it in, top it up and run it up. Follow directions after that. This whole citric acid thing has taken on a life of it's own. No one seems to report a dramatic improvement after this flush is done, so it's a crap shoot. If you use the whole 500g bottle in 9 liters you get a 5% solution. 5%, 10%, whatever. No one can really come up with a number that will effectively cut through scale, etc. Personally, I don't think citric acid is useful for this application at all. If your radiator is to the point that you think it's that clogged up, then give it to a radiator shop and let the pro's do it right. If they mess it up, then you can always come back at them. Anyway, try it, you've got nothing to lose but time and $$$. You might clear some of the crud out of the block, but a good detergent or degreaser will probably give you the same result. Changing the coolant and monitoring the health of the cooling system is probably just as effective as flushing with a weak acid solution. Just my $0.02

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325

[This message has been edited by jeffsr (edited 06-30-2000).]