Overheat once I get past 3600 RPM

[Bigboy]

I have a 2005 24"SD  Merc 350 Bravo III  -  Replaced impeller, thermostat, flushed engine, new water distribution unit.

Temp runs between 165 and 170 at 3100 RPM  once I go above 3500 my temp starts to climb over at 185, as I back down the tee  will still climb and at 190 the alarm goes off  and the smart system goes into mode. I will kill motor let cool for 510 minutes, starts like a charm, once i start moving, temp is at 165 and no other issues until I go over 3500 rpm.  I am also noticing the H2O sensor is giving me a reading between .05  and .07 .

Manifold and Risers? Replace Sensor?

Any thought would be appreciated

[wingless]

Have the exhaust components been disassembled to permit inspection of the internal raw water passages? What was observed? Is this a salt water boat? What is the fresh water flush frequency? How old are the exhaust parts? Can the exhaust be held by hand during operation?

[Bigboy]

Bought used in 2015 so I am not sure how old the manifold and risers are. Its a salt water boat.

Monday I will look at the risers to see it they are all scaled up.

But I am concerned that the h20 pressure reading at 3500 rpms does not go above .07?

Thank you

[wingless]

H2O sensor pressure? Maybe O² sensors?

FWIW, the exhaust parts are obscenely expensive and provide a short service life when not flushed, so start flushing w/ fresh water then Salt Away, or equivalent. On my custom designed exhaust I also use soapy water and drain bone dry until the next operation.

[Bigboy]

Ii am having a freshwater hookup being installed too

Thank you

[Mikeeh]

Wingless, just a question related to your flush procedure as it would relate to winter layup. An old Merc tech once told me that for long-term layup the cooling system should always be flushed and only engine anti-freeze (enviro friendly stuff) circulated through. His reasoning was that leaving it "dry" or even circulating the pink AF does nothing to inhibit corrosion and may even enhance it. So That's what I've been doing on all my boats for years. I figured I was being overly cautious but wonder if I'm just wasting money. Sorry for hijacking the thread but was wondering on other's thoughts.

[wingless]

When I winterized my boat I ran the engine winterizing fluid through the engine and the pink stuff through the fresh water / waste water. That process is gone now that the boat is in Florida.

[Mikeeh]

Wingless, that was my routine as well. Now that we live on the water with our own dock the "big boats" are gone so no freshwater systems to winterize. Thanks for responding.

[Lennyd]

I had the same issue with my 2006 240 sundeck with the same engine. Went through all the same fixed as you have and in the end I had to replace the manifolds and risers. Boat is fine now.