Your message caught my eye. I have a C18's in my boat. It took well over a year of checking everything to figure out that the culprit was a few values that were not seating right. It was minor enough that you woudl not notice in any way other than the motor would not run at normal temps. The values were letting hot exhaust gas back into the combustion chamber.
Assuming everyting is good with your cooling system it might be worth while to mention this.
United States / Canada
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