For tracking down hard-to-find coolant leaks, the best bet is to get a pressure tester. The down side is they are about $75. You can get them at Sears or some auto parts stores. Often if you have a slow leak somewhere (like a clamp) you won't see any visible leaks or white residue, but once you pressurize the system while it is cold, the leak is usually obvious.

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Steve
'93 400E