350 Mercruiser Alarm
 

I had an overtemp alarm sound as a result of a damaged seawater
impeller on my closed freshwater cooling system. I replaced it and
restarted. I was surprise that the alarm went off again when the RPMs
were increased. The temp gage was reading normal (1/4). I rechecked
everything I could think of - engine oil/pressure, outdrive lube, temp
and all appeared to be ok. The alarm will not sound until the engine
gets up to temp and then I increase the RPMs to ~1800. After it starts
sounding, it will remain sounding until the engine goes cold and is
restarted.

My questions a
1) Besides the three items listed above (oil, temp and drive lube) is
there any other thing that will cause the alarm to sound?
2) If I mistakenly mixed the sewater hoses (inlet ond outlet) would
this cause the alarm to sound and would the temp still show "normal"?
3) After I replaced the impeller, the first time the alarm re-sounded
was when I was attempting to adjust the trim, any relation?

Any ideas, inputs or suggestions would be greatly appreciated,