I checked out the VesselView and it's Mercury only, but Volvo Penta has a version also called Easy Connect.

Seems at lower rpm younhave sufficiemt cooling. After planing throttle levels, it begins to heat up.

Do the temps stabilize at 1/2 and full trottle?

Once back to idle and light throttle it takes 10 minutes to drop back to rated thermostat temps. Once dropped...it can maintain the 140°F?

10 min is a long time to drop temp with raw water.

Where is your water temp sensor located?

What is the avg temp of your raw water source? As in the temp of the lake you run?

Lake temp is 59 degrees. Seemed like a long time to cool back down. It’s a 140 thermostat but temp gauge reads stable at 150 when idling, even for prolonged time.
When advancing throttle to half throttle and planing, it held steady for several minutes then increased slowly to 170. Temp increase seemed to slow down at 170, maybe barely creeping up after 170. Advanced to full throttle and after a few minutes it rose to 190-200 and alarmed/derated. I cut it back to neutral and idled but it took longer than expected to cool back to 150. Hope that better explains it.

I pulled hoses off the exhaust manifolds today and water quickly and freely drained out.

I'm going to replace the plastic nipple going through gimbal although I’ve inspected it end of last season.

There are two water temp sensors, locate within about 12” of each other on the thermostat housing and crossover tube.
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Also going to replace the intake hose from transom to PS cooler, although it feels very stiff and unlikely to collapse. I’ll look through the through-transom pipe to check for corrosion but I didn’t notice any when replacing the transom to gimbal hose.

Temp senders
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Was curious if the half throttle would equalize temp vs water flow etc. Of if it would still have an eventual thermal run-away.

You're pumping water at a reasonable volume at half throttle. Where is the heat coming from? Either the engine is making more of it, or your water flow is restricted. You've changed almost everything.

How is the engine performing? No change?

Are you running a thermostat? What temp?

Have you tried a high volume thermostat?
Or perhaps a restrictor plate instead of a thermostat?

Also, do you have equal water amounts coming out of each exhaust tip (assuming through hull). If so, is water coming out the same temperature?

Engine runs great. Spark plugs indicate rich condition but are uniform. Driving me nuts. Gonna pull transom to PS hose tomorrow and check PS cooler and through-transom tube, but it’s not like a bravo setup so shouldn’t be a problem. Maybe the intake hose is collapsing at high raw water rpm’s.

Running a Volvo Penta OEM 140 thermostat. Not sure if there’s a high volume option. I may pull it out and see if it still overheats. Boat has a Corsa captains call exhaust. Water only dribbles out side ports when solenoids activated. Comes put transom exhaust port and overflows when solenoids not activated.
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Video is too large to post

Yes, he replaced the hose assembly in the transom assembly. I would definitely hook up a temporary water pressure gauge. There may be a port on your intake or t stat housing that you can use or if not, you can use one of the block drains. https://cimg5.ibsrv.net/gimg/www.off...a6dca2ce65.jpg
My guess is your issue could be the hose circled in red. The hose my friend replaced was essentially this hose in a Bravo transom assembly.

That hose was OEM new last year. The lower nipple will be replaced today. Will inspect upper through-transom pipe today. All tstat ports are used. Block ports have drain system but maybe I can plumb a gauge into end of that.