3.0 Overheat issue that I can't diagnose.

I have a 98 Sea Ray 180 with a 87 3.0 mercruiser (my first boat). I just put a reman head on it and a year old manifold and riser. I have replaced the raw water impeller and the thermostat. My issue is the engine stays cool at 130 while at idle and at no wake speed once on a plane after I back off to cruising rpm it steadily heats up to 170 hangs for a little and then increases to 220 where I pull back to idle and let her cool and then I shut it down (to do a little fishing). Now the weird part is we were late getting home one night and I just got up on a plane and left it in WOT and it went the whole width of the bay at 170 and never got any hotter. I was suspecting the impeller again even though it only has mabey 10 hours on it but if it was the impeller wouldnt it either overheat at idle or WOT? Thanks for any help!
Nick

Re: 3.0 Overheat issue that I can't diagnose.

Have a look at this Mercruiser service bulletin on overheats above 3000 rpm.

Re: 3.0 Overheat issue that I can't diagnose.

Thanks for the fast reply Don. I am going to be on the boat tommorow so I will deff do this and report back the results. Just so i understand, from reading this it is seeming like (if this was the problem) that when I installed the impeller I might not have gotten a good seal and am drawing in air?