Sorry - I misread your "fresh water" comment as meaning this was a
"fresh water version" of the engine, which I believe Yanmar did make.

I've heard the that the raw water versions have the thermostat set at
either 125 or 140 degrees, but I don't know for sure which is right or
whether there's a good alternative. You could call Torresen, who
certainly has experience running Yanmars in fresh water.

OldSailor wrote:
"Jeff" wrote
I have a pair of 2GM20's on my cat. I added two water temp guages
purchased from West Marine - they're pretty generic. The block has
several places that you can add a sensor. For instance, there should
be taps for sending the coolant to a hot water heater.

The 2QM15 is a raw water cooled engine so no water heater taps, but there
are a few other plugs. Problem may be getting them out without damage after
almost 30 yrs?

You can also simply add a tee with the alarm.

If alarm is in a dead end rather than in water flow, I am a bit concerned it
may not see actual temperature and may collect scale, but worth looking at.
There is a place where block water exits head before going to manifold via
rubber hose - I could put gauge sensor there, but it would be neater if I
could use one sensor for gauge and alarm.

One of my engines runs at 165 to 175, depending on the load from the
big alternator and the heater. The other, without these loads, runs
at 160-165, depending on how hard I push.

I thought the fresh water thermostat was set at 175, but my books are
all on the boat.

Presumably you have closed circuit fresh water cooling - We are once
through. But, because we are IN fresh lake water, scaling should not be a
problem and I hope to get temperatures up to those that you see. To do
this, I will need a new thermostat and possibly temperature switch. Yanmar
don't seem to have these specific parts listed for my engine, but perhaps
parts from other engines may fit?

Also, I second the advice to get the Seloc book. Also you should have
the shop manual and the parts books.

I already have the owners manual and illustrated parts book - I ordered the
Seloc manual about a week ago without ever having seen one - Glad to hear it
might be useful. Some of those generic manuals are not that great! Shop
manual may be worthwhile, especially if I ever decide to rebuild engine
myself.