This weekend, went to leave a cove we were in and the anchor was stuck as hell. so i pulled it a bit w/the boat to free it. it was a bit of a struggle, and i didn't notice it immediately, but when we went to get up on plane the boat would bog down above idle. i then noticed my temp was WAY up. so i shut it off, opened the cover and let it cool. after it cooled a bit, restarted it and it ran fine for the rest of the day. then when putting it on the trailer i noticed it did it again. just the little bit of resistance of running it up on the trailer caused the temp to run up real quick.
i'd think if it were the impeller, wouldn't it get hot at idle too. 'cuz it doesn't. only when there's some kind of resistance on the boat in the water. heck, maybe it's always done this, i've just never noticed. BTW, it's time to change my impeller anyway. (been 4 yrs)