It seems to me that everyone knows the problem (acid) but no one has told how the average person can test for this acid. Well, here is a little trick I learned a few months ago. Set a digital meter to mv and put the positive probe in the coolent and the negative probe to ground (engine, body or battery). If you get over 300mv (.3v) then change your coolent. If it is under 300mv than everything is fine. I would still change the coolent every 2 years but the DVM trick will help if you forget, or if you aquire a used car and don't know the history of the car.
I didn't believe this at first but after doing some testing on my own cars (some maintained very good and some very bad) I proved to myself that it works.

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84 500 SEL (307,xxx miles)