I can't site any specific thing or engine part to look at/repair/replace but
your evaluation of a "vaporlock" that is thermal related is a good lead.
You can only tie down the actual cause when you can monitor the water flow
at the temperature it happens. When you wait for it to cool enough to
handle the thermal blockage has already changed and relaxed it's blockage.
What is needed is to install "Flow View Sight Glasses" in the water line(s)
to be able to visualize the amount of water flow at all times that the
engine is in operation and at all temperatures. At least one wide view sight
glass cataloged in Grainger's is rated for 130 PSI @ 212° F so that they are
safe to install in the hoses of your engine and come with standard pipe
thread connections in sizes ranging from 1/4" up to 1-1/2" with a cost
ranging from $93.30 up to $197.25 each. I would put one in either the water
pump supply or discharge line and a second one in the hose between the
engine and the exhaust manifold connection.
Where to look with in the engine are the two heat producing areas 1.) the
engine Block/Head and 2.) the exhaust manifold as mentioned in another
thread response.
Good Luck with this engine.
Jim


"Jeffrey P. Vasquez" wrote in
message ...
Hello all,

It's me again and I've a new riddle to pose to you. It's the S2 with the
raw-water cooled Yanmar in a marine environment again. Here's the latest:

1) Through-hull has been thoroughly cleaned.
2) Raw water strainer and hoses have been replaced.
3) Water pump has been replaced.
4) Thermostat has been replaced.
5) Mixing elbow has been replaced.
6) Entire system has been "Marsolved" (marine solvent)

Here are the symptoms:

When the engine is first started there is a good flow of water through
the system as evidenced from the exhaust. The engine runs to temperature
(150F) and operates fine indefinitely with no fluctuation in temperature
at operating rpm.

After operating at temperature for some fifteen minutes (arbitrary, but
approximately the time it takes to clear the marina and channel), the
engine is stopped and allowed to cool. After cooling the engine is
restarted and no flow of water is observed as evidenced by the exhaust.
Running at idle or at operating rpm does not alter the condition and the
engine heats to alarm. This has been confirmed in absence of sailing,
i.e. a siphon break was suspected (and may have been a past culprit), so
the hoses were rerouted to ensure positive pressure to the water pump
when the boat is level.

If the system is disassembled upstream to downstream (after cooling
sufficient to touch), flow through it at all points is observed. If the
system is reassembled it will at this point again function properly.

I would liken this behavior to vapor lock in automobile fuel systems, but
I have no understanding how this would be relevant to the Yanmar cooling
system.

The mechanic's diagnosis is blockage in the engine block itself and the
suggested recommendation is replace the entire engine.

Any opinions, experience, recommendations or wild-eyed guesses would be
infinitely appreciated as always. If engine replacement is the correct
course of action, what does one do with a Yanmar 2GM with about 1800
hours and a suspected cooling blockage?

Many thanks!