the impeller that's installed now, change to one with more blades and check it again. It has to be something simple because I have checked and changed everything on the engine during the rebuild a year ago. It was heating after the rebuild in Mexico as well so I installed a second heat exchanger an that solved the problem. It never heated up in Calif. waters in the 10 years prior to heading south. It didn't heat up last year when we returned to Calif. and removed the second heat exchanger because it was running to cool. This time in Calif. with the single heat exchanger it's heating up and the only thing different is the numbers of blades on the impeller. I think....