Have you tried to remove the little plug on top of the heat exchanger and pour antifreeze in through there. It could take up to a cup or two. I had a similar problem and it turned out to be an air lock in the heat exchanger. Somewhere on the top of the exchanger is a quarter inch plug take that out pour antifreeze in until it is full. The other thing I did was flush out the raw water side of the heat exchanger with the dock side water. I disconnected the hose after the impeller and took the farthest away hose I could on the exit side of the heat exchanger and ran fresh water through it. There was the smallest piece of rubber that came out and the gen never over heated again.