End caps are off - only a few tubes required cleaning. The seal on one end was in pretty bad shape with holes and just wore out. I ordered two new seals and they recommended I use radiator flush and run trhough the heat exchanger. Its off the engine so I don't know how productive that will be. Also, replaced the coolant sensor while I was down there.

trying to drain coolant from block...I think I've located the drain on the side of the block where the power steering pump is located. Is this the only one and does it require a special allen wrench (size?).

found them....looks like 17 MM hex on front bottom of block

No, they are near the rear of motor mount. One on each side. Just above the oil pan mounting surface. On the side with the starter I had to move the single point drain valve to see it.

They're 18mm hex plugs that have an allen pattern also in the head.

found the port side, but just can't see the one on the side with the starter. I must be blind!

Like I said I had to undo and shift that drain valve out of the way. It's almost behind the oil dipstick tube but essentially in the exact same spot on the block as the port side.

Ok, this may sound dumb but have you tested your thermostat? I was having very similar issues and found my thermostat was barely open at 170 and not fully open till 195..and it took forever for it to fully open too. This was a 160 degree thermostat. Pretty much the same issue though-ok at idle and low speeds but when you start pushing it then it slowly gets hotter and hotter-shut it off for a couple minutes, start it back up and temp would drop to 150 and then repeat the whole cycle again. Brand new impeller with no pieces missing on the old one and system was back flushed.

Maybe this will help a little. Test the one you have now.

Dave

Thank you, I replaced the thermostat, flushed the engine and heat exchanger, added new coolant. The impeller is new (2 weeks). The boat did run cooler, but when you run it up to 48-50 RPMs still creaps up towards 190. I did not let it get to that point. Runs great at 50 mph.

Salt water boat - does it have a bravo one drive? could be the dreaded bravo one cooling hose restriction as mentioned above. My boat had the same symptoms.