Mercruiser 8.2L Overheat

Have a mercruiser 8.2L with the closed cooling system and hurth transmission. Bought the boat with 148 hours from a dealer. Previous owner had the boat with full warranty and never reported any issues. Impeller was changed recently due to Mercury recall.

Symptoms... Boat stays at 170 when at idle and when revved up in neutral. Will randomly overheat when wakeboarding or with ballast or not. As in I can have someone ride for 10 minutes and everything is great then they will fall and I'll start again and the temp will shoot up. I can then put it back in neutral and rev it and the temp comes down.

What I've done... First thought was impeller so I changed it, the old one was perfect but it's a bitch to get to in this boat so I changed it anyways. Had the heat exchanger pressure tested and it checked out fine. Replaced the thermostat with new 160*. Coolant is full and flushed with new dexcool. Checked trans cooler screen and it's free of debris. Tightened all hoses in case of air leaks.

None of that worked and I was still getting the overheat issue so I admitted defeat and took it to a shop.

They have confirmed good water and coolant circulation.

Now here's the kicker, and where I'm confused. This morning they called and said they discovered water in my transmission oil. I'll admit I don't check my trans oil every time I take it out but I have in the last 5 hours and it didn't look milky to me.

The tech believes this may be what's causing the overheat. Would the trans temp have anything to do with the engine temp, or could the sensor be tied in somehow? I've never noticed any transmission issues like noise, clunking, or slipping.

The trans has now been flushed and we're taking it to lake test with the tech on Thursday but if anyone has any ideas between now and then it would be greatly appreciated.

Where the hose comes up from the raw water pump to the heat exchanger, check to make sure there wasn't a blade off of the impeller stopped up in the heat exchanger. This could have happened before you boat the boat. It will still pressure check just fine, but will not flow water. What you can do is take the intake and exit hose off of the heat exchanger and take a water hose and back flush it to see if anything comes out. Then flow the hose both ways to make sure your getting the same amount of water in and out on both ends.

Checked that and it's clear. Also should mention that the dealer I bought it from did all previous maintenance on the boat since it was new in 2012 and never had to do anything other than the recalled impeller and normal scheduled maintenance/winterization.

We had a customers boat last summer doing about the same. We would go out and run it and before we could get from the slip to the break water the engine high temp alarm was going off. We checked just about everything and found that whoever dewinterized it forgot to hook both knock sensors back up. Long story short it would start up and run fine and the ecm would advance the timing like it should but never picked up any detonation so it would run the full amount of timing it was programmed to run. The results were too much timing for the air and fuel conditions and it would overheat. Plugged them back in and never had any issues after that. It may not be the same issue your seeing, but that would be something easy to check. Other than that with as new as yours is, I'm not sure what it could be unless the put the wrong impeller in it from the recall you mentioned. Did they change the housing and all?