Requesting help to determine why 2022 25 lsv M6di engine overheated. 75 hrs on boat. 2 yrs old.

Ride was 45 minutes. Steady 25 MPH. When reached destination and idle speed, audible alarm sounded.

 Display: Engine Coolant Temp High.

Steps taken:

1: Pulled impeller.  It was fine. Replaced anyway

2 Checked strainer.  It was loose.ish  NOT hand tight.  Moved when put my hand on it tighten.  Strainer clear but plastic has visible spider cracks (likely front winter freeze). Took off, filled with water, no leaks.  Could leak under pressure I suppose.

3 Coolant reservoir empty

4 No evidence of coolant spray under plastic engine cover.  None on top of plastic valve covers.  small pools of coolant on engine block on front side of engine between belt and raw water impeller.  Also evidence of coolant pool on side of engine by heat exchanger.  Checked all hose fitting.  No movement and clamps were tight.

Questions: where does coolant go when it gets hot.  No evidence it came out of the reservoir cap.  Heat exchange maybe?  Where is the pressure valve?

Dealer appointment set for next week but I like to fix things if I can.

Side bar question.  What is the plastic cover labeled "RC".  It was laying in the Blige area.  Boat serviced by 2 dealers in last 3 months.  1 where I bought it and 1 local.

https://photos.app.goo.gl/RfPKPzifEKwb9uTh8

https://photos.app.goo.gl/sWSZCcH71cF5e6WPA

https://photos.app.goo.gl/eaPFx9gfhdx7QeA98

41 minutes ago, Btown801 said:

This is what Christensen oil in Utah lists the fuel as:

100 Octane Race Fuel

An alternate solution to using additives is to use 100 octane Unleaded Racing Gasoline. This is a street legal 100 octane unleaded gasoline that can be used in its pure form, or it can be blended with any street gasoline. You will always know what octane you end up with because we can provide you with a blending chart that helps you to determine that. In addition, if the 100 octane gasoline is used in its pure form, the engine will make additional power due to the "improved burn" characteristics. This is because 100 Unleaded Racing Gasoline contains a very select group of hydrocarbons that vaporize and burn much more readily than those found in conventional street gasoline. When more of the gasoline is burned in the combustion chamber, the engine makes more horsepower. This phenomenon is known as improved combustion efficiency.

Should work.  100 octane race gas - like $20 a gallon?  Is there no premium fuel on that lake?

Another option mix in an octane booster:
https://vpracingfuels.com/product/octanium-unleaded/?c=207&

This is what Christensen oil in Utah lists the fuel as:

100 Octane Race Fuel

An alternate solution to using additives is to use 100 octane Unleaded Racing Gasoline. This is a street legal 100 octane unleaded gasoline that can be used in its pure form, or it can be blended with any street gasoline. You will always know what octane you end up with because we can provide you with a blending chart that helps you to determine that. In addition, if the 100 octane gasoline is used in its pure form, the engine will make additional power due to the "improved burn" characteristics. This is because 100 Unleaded Racing Gasoline contains a very select group of hydrocarbons that vaporize and burn much more readily than those found in conventional street gasoline. When more of the gasoline is burned in the combustion chamber, the engine makes more horsepower. This phenomenon is known as improved combustion efficiency.

23 hours ago, CaptainMorgan said:

And wouldnt believe any math based on fuel tank gauge - mine is often off by 1/4 tank or more

Ya my T220 tank was idling to the trailer at 2% gas and only took 39.5 gallons despite the 58 gallons capacity. I expect some safety/gas design past 0% given how the tanks slosh around, but not 18 gallons worth! 
 

we seem to be using 10 gallons each 75 min or so surf session fwiw, very rough approximation on a m6 t220