More information would be helpful.
Assuming you have a 6LPA-ST series?
Do you have a straight inboard, or an I/O?
Are you the original owner, or did you inherit the problem?
What does 'peg the temp' mean? The alarm
comes on or does the gauge read 250 after 30 seconds?
Did the problem start all at once or come on gradually?
Can you modulate the temp with speed, or does it overheat at anything above idle?
What is the ambient temperature and condition of hull
and running gear
Based on what you've said so far, I'd guess, in no particular order, overpropped, airlock in the freshwater side, switched hose, faulty freshwater pump, reversed thermostat, seriously dirty hull
, flow restriction after the riser.
Couldn't find a good cooling
schematic, so the above are somewhat obviously all guesses....