Join Date: Jun 2004
Location: West Michigan
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The expansion tank also serves the purpose of seperating air bubbles, it has to be high enough that the air in the cooling system will migrate to it. On a car the top of the radiator will be above the highest water passage in the engine, so it does the same thing.
Too low, and the bubbles will form in the exchanger or the engine, and reduce heat transfer which can cause the engine to overheat.