M-25XP Running Too Cold?

[Sail Savannah]

First post, long time reader. Learned a ton from you all! I bought an '88 Catalina 30 MKII 2 months ago, put about 80 hours on her. It came with a 3" heat exchanger that looked to be on its last leg. Always ran around 165 on the gauge, just replaced it with a Mr. Cool model.

First time firing her up, it overheated. This wasn't surprising as I figured I'd need to burp the lines. I used the method I'd read about on here and filled up the line on the bottom of the thermostat to remove the air. This is where things get interesting. Started the engine, put it in gear at the dock for 20 minutes at 1500rpm, and the gauge never read more than 140. Normally at this point it'd be up around 165. I shot the thermostat housing with an infrared thermometer and it showed 145.

My understanding is that the thermostat is 160 degrees. So at 145 the thermostat would be closed and the heat exchanger irrelevant, how is it staying so cool? And, more importantly, is this a problem? It's under the operating range in the manual, and I'm afraid it won't get hot enough and lead to carbon buildup over time. Could the thermostat be stuck open? I also ran some tests on the temp sending unit but it seems to be fine (pinned to the right when the wire is grounded, around 275 ohms at 140 degrees).

 

[RoyS]

At 1500 RPM your engine is not working very hard, even in gear. Your water heater may be acting as a heat exchanger as well. You can remove and test your thermostat, but I suspect that you have no problem.

 

[Benny17441]

When in doubt, go ahead and verify that the thermostat is not stuck open.

 

[Stu Jackson]

Started the engine, put it in gear at the dock for 20 minutes at 1500rpm, and the gauge never read more than 140.

That is NOT a problem. You MUST take the boat out and run the engine under load. I have often run m y engine at anchor for charging for an hour at 1500-1800 rpm and it barely gets to warmup - again, that's an entire hour. Alternatively, run it in gear and cruising speed at the dock, tie her up well!
You do NOT have a problem.

Engines 101 - The BIGGEST & BEST collection of M25 Series Universal Engine Information on the Internet, plus some M35, too

Diesel Engine - c34.org

 

[Sail Savannah]

Thanks for the input everybody. Had a chance to take her out today and after running at 2300rpm (my typical cruise) for about 30 minutes it was running at 165. Definitely took longer to warm up than the old heat exchanger, so not sure if that could be a thermostat or just a more efficient heat exchanger, but like Stu said, not really a problem.

Stu - thanks for the link, that's a fantastic resource.

Benny - if I ever want to check if the thermostat is stuck, does the housing have to be removed to check?