1991 Mercury 75hp overheating at idle even after impeller replaced

[mlbinseattle]

On mine, and I was lucky to even notice it the first time, there is a guide pin that rests on the driveshaft, and it is on that part of the shaft inside the impeller housing. The shaft has a flat spot where the pin or key (whatever it's called) sits, and this pin locks the impeller to the shaft. When you put the impeller housing in, that key must be in place, or the impeller won't turn or won't turn properly. It's the pin rests on the flat spot, and its a little tricky to get it to stay in place while sliding the housing down. I used a dab of multipurpose grease on the flat spot keep the pin stuck in place. I bet this is your problem. The pin is probably missing.

If you're sure that the pin is there and the impeller is turning properly and not slipping around the shaft, then it sounds like a clog. You may have a piece of old impeller stuck in the head, somewhere. I had to replace all my gaskets, and I noticed several pieces of old impeller, although small but big enough to clog a passage, in the block. HTH's.