"Calif Bill" wrote in message
news:1Lekc.17663 Some of the engines will get an air bubble up high in the
engine (Merc
Optimax) from long idling down the channel when first starting out. There
builds up a bubble on the upper cylinder waterjacket. Upper cylinder
overheats. So put it up high. If partial failure of the impeller and low
flow, H2O may not reach the upper cylinder.


I have seen this approach backfire.

The sending unit wants to measure the temperature of the water. If there is
no water, then it will either measure the temperature of the air/water vapor
or of the fitting that it is bolted into. The sending unit is usually
brass, which conducts heat pretty well. Maybe not on an outboard, but most
inboards what you screw it into is cast iron, which does not conduct heat
very well. The water to brass heat conduction is very good, much better
than air to brass, so when there is water the sending unit will make a
reasonable measurement of its temperature. If there is no water there, then
the sending unit will measure something between the block temperature, the
inside air temperature and the outside air temperature. That's kind of hard
to predict what the result will be.

Rod