Thanks for the info, it sounds like an acid flush is the way to go. By the way, when it almost overheated, the dial was at the white line which is just below the red line, right around 120 degrees. Once we replaced the aux fan fuse, it cooled down to about 90. The AC was running the whole time. The owner said he never had it go above 90, but he rarely drove it, and when he did it was on the highway. When I test drove it I took it around town and drove it kind of hard to make sure there weren't any existing problems.

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87 300D - Running on Veggie oil; 260,000 plus miles; Original #14 head