[C320-list] Engine overheating

[Colin Evans]

Hull 774,   2000 model
Over the last few months the temperature gauge has been reading a bit higher
than the normal 160 degrees, up to 180.  .  Plenty of water is pumping
through and the raw water filter is clean. Coolant reservoir is full to the
mark.
Yesterday it went to 240.  I shut down the engine , then re-started a few
minutes later and it went back to 180 and stayed there for a while then
jumped to 240 again.  I shut down the engine again and the same thing
happened.  Back to 180 degrees.  The water warning  light didn`t come on.
The engine compartment didn`t seem particularly hot.  It was a hot day,
about 35 C (95 F)
We anchored for an hour then motored back to the marina , about 20 minutes
and the temp never went above 180.  While reversing into the pen the temp
dropped to 170.  
Any ideas, sender or display?
 
*************************
Regards
Colin Evans
Watermark 
Hull 774 
Lake Macquarie, Australia