Thank you for that insight.
Yes, my typo = 3126b - inline 6 - turbo.
And it is true that I do not push the boat much. 45' single engine trawler, and in no real hurry. Run at 1500-1600rpm when traveling. Never been to full throttle, maybe 2,400?
I do not notice power reduction once up to temperature nor have I ever seen black smoke.
I appreciate the suggestions and will look into getting a compression test as next step. Thanks!
I'm in New England. Using the block heater does help, even in the summer. Recently had 1k hr service performed, but did not eliminate it.
A compression variance was never mentioned.