Underheating Engine

[Hud Smith]

We finally got the boat (CD30K) out today and motored around to check the engine.
Normally the temp gauge moves up to about half, then the thermostat opens and the temp drops all the way down to minimum- the thermostat then closes, and the temp rises again. This cycle goes on continuously, every 4 or 5 minutes.
Today the temp gauge rose to about 33%. I increased RPM's to 1900, and the thermostat opened, dropping the temp gauge to minimum.
However, the temp gauge never rose again, despite hi RPM- it stayed at minimum temp. I was able to put my hand on the valve cover without much discomfort.
The exhaust water was warm to hot, but I could hold my hand in it with no discomfort.
I'm thinking that the thermostat (installed 2 years ago) is stuck in the open position.
I would really like to avoid getting under the cockpit to replace it again.
Does anyone have any ideas about how to loosen it up without removing it?
Thanks for any help.
Hud

[Jim Lewis]

When my MD7A temperature cycles up and down from mid range to cold.....that has told me heat exchanger in need of cleaning or overhaul.....if runs cold all the time thermo could be open and not closing.....possibly engine passages could be partially blocked too.
I use mine regular and about every ten years need to clean out the system

Good luck