Mercruiser 165 over heating help

I checked the thermostat seems find and disconnected the hose going to the motor from the stern drive and lots of water coming in the hose from the thermostat going to the block on the left looks like a u side ways is extremely hot but everything else seems cool and warm.
I drove for 5 min it seemed ok ran between 145 to 165 degree then after 15min it climbed all the way up to 200 and climbing what could be doing this. I'm new at this
I've heard some say flushing the motor might have flakes and particles plugging the water to go thru but how would I know is there test I can do to find out and how to flush to clean it out it's been sitting for a while
1977 mercruiser 165 6inline fresh water use only never been in seawater

Re: Mercruiser 165 over heating help

Could your impeller that's giving enough water...

Re: Mercruiser 165 over heating help

I have a pair of 165's in my boat, and overheated one a couple weeks ago. It sounds like the thermostat isn't opening. It should be open at 140 degrees (you can allow about 20 degrees for guage errors and such). It opens to let hot water escape to the exhaust manifold, and that lets cold water from your impeller (the hose coming from the transom assy) into the engine. If you run it on the muffs you should have hot water coming out, alot of it. But thermostats are cheap. About $15. Gasket included. Change the thermostat just because. A bad one looks just like a good one. The only way to test one is to cook it and see when it opens. And that still won't tell you for sure. It may be binding intermittently.

So depending on how hot the exhaust got, you may have melted the water pump in the drive. It is plastic, and in the path of the exhaust.

You can also take the hose loose where it goes into the exhaust and see how much water is coming out. And my last comment on this is to dig through this site. One of the guys has an alternate hose routing setup that is supposed to keep things cooler. I haven't tried it, but I may sometime.

Good luck.
-Jim

Re: Mercruiser 165 over heating help

i agree with the thermostat, one thing you can check to see if the impeller is working ok is take the hose feeding the engine (it runs from the back the engine)take the hose off and make sure your muffs are on and water flowing and start it it should come out like a water hose and i mean fast if not you might want to replace it,, i have a mercruiser 120 that did the same thing another idea is take the thermosat out then try runnung it and if it over heats then its not the thermostat just a thought,, good luck

Re: Mercruiser 165 over heating help

Take the thermostat out for now and try it. If it's ok then replace it. Remember, spring side down. You will need a thermostat that far north Can...