I have a 2001 Bayliner with 3.0l Mercruiser. It has an aftermarket Seakamp heat exchanger. The boat ran flawlessly until last summer when it would start to overheat when I’d slow down to no wake speed or throw it in neutral. The temperature will continue to rise until I either idle up to a couple thousand rpm or get her up on plane real quick and then the temp will drop right down to 160 degrees. The boat doesn’t overheat at speed, just when I’m idling into harbor or have to drive real slow. Other than the overheating it runs flawlessly.
I have put a new impeller in it, the one that was replaced was in perfect condition. I have also put a new circulation pump on the engine to circulate the freshwater side of the system. I’ve pulled the exchanger to see if it was clogged and there are no obstructions or corrosion inside. I’m stumped!
Anyone have any suggestions for me before I take it in to a shop? I live in W Seattle and was also wondering which shops people would recommend. I’d be willing to take it anywhere from Tacoma to Everett.
I would appreciate any help offered here.

Thanks, Dave