Mercruiser 140 hp. used regularly, maintained VERY well. During an outing, temp gauge rose from its normal 150 degree position to 220 before it was noticed and shut down. The following remedies were preformed one at a time. Both water pumps replaced. Temp. sender and gauge have been replaced. The use of a hand held infa-red thermometer shows the entire block, manifold, and all but one spot on the head (@ the #1 exhaust port) at 150 degrees. The hot spot rises to over 200 if you let it. So, then the head was inspected, resurfaced, and has new stainless valves. If one didn't know better you'd say it was in perfect condition. BUT, after running approx. 5 minutes the hot spot reappears on the head, this time between the #2 and 3 exhaust ports. The rest of the engine remains at 150; the gauge reads 200+. WTF?