Mark: Don't jump to the conclusion that your 2GM does not have a heat exchanger -- many do. If there is a Sendure heat exchanger, it will be located aft of the engine, be made of brass, be about as big as loaf of bread, and have an automotive radiator cap. They are very easy to remove. Yank it and take it to radiator shop to be boiled out and inspected. The end caps and gaskets will probably have to be replaced. While you're at it get a couple of spare zincs.
