Hi everyone. New member here. I've read this post with interest as the same issue plagues my 2012 150 Speedster 260. Last fall, I noticed that the engine coolant temp rose to 208 degrees when traveling above 6500 RPM. When i slowed to about 5500, the temp steadily came down and fluctuated between 188 and 190 as it has since I had the boat new. I did not push it again above 208 but I think it would have gone higher given the way it rose steadily.
I tested the anti-freeze properties prior to storage (Southern Ontario, Canada) and found it to be acceptable for the winter. My plan was to replace the coolant in the spring. The following is what I've done:
Drain and flushed the coolant with distilled water, with engine running.
Old coolant had some crud in it but ran clean with the first flush.
Flushed with water 4 times.
Replaced coolant, 50/50 mixture and purged the system of air.
Coolant reservoir tank levels are spot on cold and hot.
New coolant tank cap.
Still runs hot over 6500 RPM.
Drops right down to normal (188-190) below 6500.
Boat is always used in fresh water.
No water or coolant in the bilge, it has always been dry since new.
Performance of the boat is still great.
No white smoke.
No coolant in oil.
No coolant loss.
New plugs every 100 hours.
New oil and filter seasonally (50-60 hours).
295 hours on engine.
Supercharger rebuilt at 195 hours (as per routine maintenance).
The thermostat seems to be working fine as it opens and closes right at that 188 degree mark. I've read about the other repair attempts and believe I have the same issue. The one thing I have yet to try is a chemical flush of the cooling system. There may very well be some flow issues in the ride plate. That's the only thing I could think of. Does anyone have a brand of cooling system flush with which they've had good luck?
Any other suggestions wold be appreciated.
