On Sun, 25 Nov 2007 21:37:55 -0500, "Roger Long"
wrote:

My 2QM20 has a thermostat. It is also a very cool running engine. I have
heard several places that they are among the coolest running diesels in
common use and mine seems quite happy running that way. I have also heard,
but not substantiated, that this engine runs just as well, or better, with
the thermostat removed. I don't recommend it or plan to do it though.

The 2QM20 has a precombustion chamber which I don't think is common in small
diesels. This might account for different operating characteristics. It
might also account for their reputation for lasting a long time with poor to
non-existent maintenance. With a precombustion chamber, a clogged injector
maybe isn't consentrating the combustion into hot spots on the piston head.
I'm speculating a bit and would like to know more however.

Yanmar's factory parts books lists two thermostats and two overheat
sensors for the small engines. One for salt water cooling and a higher
temp one for freshwater.

Also all the small Yanmar engines and I assume the larger engines use
pre-combustion chambers as do Caterpillars and many other engines. The
idea of the pre-combustion chamber is that the fire starts there and a
gout of burning fuel is injected into the combustion chamber rather
then simply spraying a jet of fuel into the combustion chamber. I
believe it is intended to provide better flame propagation and
improved combustion.

I would hesitate to recommend that anyone remove the thermostat unless
the engines is equipped with a temp. gauge as in some cases removing
the thermostat actually increases cylinder head temperatures.

It seems contrary to logic but on some engines - there was a Ford V-8
I remember - that actually run hotter without the thermostat as the
thermostat in addition to controlling temperature acted as a orifice
and restricted the speed of coolant flow through the block and head.


Bruce-in-Bangkok
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from address for reply)