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Working on a Volvo gen 8 300 hp 2017 closed cooling . Replaced impeller thermostat and motor is still running hot . Not a lot of flow out of out drive ext. less than 200 hours and flushed after every use. I am thing risers any thoughts. Thanks

Not really that intimate with that series motor, but here are some general suggestions:
Did you try removing the freeze plugs on the manifolds/risers and flush through those, poking something into the ports to clear any debris?
You could also remove the hose off the sea-water pump and try a manual reverse flush through the intake side to see if there is any debris clogging the take-up side?

Are the heat exchanger tubes clogged up?

Yes tried both heat exchanger clear removed plugs and flushed not coming out on ext side

Have you pulled the anode access plates on the exhaust manifolds. If you are thinking risers those anodes should be pretty chewed up.

Yes we did and replaced the risers. Operating at 165 to 169 at full RPM. What is the operating temp for these engines Cat converter 5.7 2017 model year.. Thanks

That is where I am at on a 2021 350 VP

I run 162-165 on the 2016 version. Water temp seems to make the difference.