I have a Mercruiser O.E. service manual. I cannot find any procedures
or specifications for checking coolant flow at any point on the engine.
Is there a test port? If there is, does anyone know the
specifications related to coolant flow? I am going to go ahead and
replace the impeller located in the stern; however, I have a sneaking
suspicion that I will remove the unit and find no damage. My factory
service manual does not provide a test procedure for the engine temp
sensor. Does anyone know what it is. I would assume that it is likely
a resistence test. Before I go to the length of dropping the stern
drive, I want to rule out all other possibilities. I really appreciate
all of the advice. I had an impeller fail on a boat with a 5.7 L Bravo
set-up, and it was not like this. It was obvious no water was being
drawn up at that point. Like I mentioned, when I removed the
circualtion pump and hoses water poured out from all directions.

Water pouring out doesn't mean much. If your exhaust is plugged or
restricted, cooling water can't come into or leave the engine. A
defective water pump could push enough water up to the engine to fill
it but not produce the pressure to pump the necessary volume to cool
the engine. There is no spec. that I know of, but I would guess that
the stern drive pump should pump at least 5 gal. per minuite at idle.
Jim