The coolant flow can be checked by inspection almost immediately, though a
partial clog can look OK and still send it into Overheat. Certainly I'd look
there first if there was any significant change.

However, if the gauge is moving quickly, like up and down in a matter of
seconds, it can't be a real temperature issue, because the coolant can't change
temperature that quickly. I've never had a temp sensor do that, but I've seen
it from a faulty oil pressure sensor, and I've seen it from a dangling wire.

When my heat exchanger was partially clogged the engine was more sensitive the
overheating problems. I could tell within a few seconds if my wife turned on
the microwave because the temp would start to creep up. Now the temp is a bit
more stable, but I can't run the engine up to 3000 rpm if the alternator is
fully loaded.

BTW, last week I did have an overheat situation: within a minute after dropping
the mooring at Kittery, I noticed the starboard engine temp going up. It put it
into neutral and asked my wife to check the water flow - there was none. We
killed the engine and went back to the mooring (its sometimes nice to have two
engines!). First I pulled the intake of the pump - full flow meaning no intake
clog. Then I pulled the output hose off and asked my wife to "start" with the
kill switch pulled - no flow. At that point I just pulled off the water pump,
removed the cover (damn Yanmar mounted the pump on the GM's so that the cover
can't be reached without pulling the pump!). The impellor looked OK, but my
wife asked how it worked so I spun the pulley to show her but the impeller
didn't spin! I seems the impellor was spinning free on its hub. A spare was
put in and we were underway again with a total delay of about 45 minutes.




"Capt. Mooron" wrote in message
...
Good Grief Jeff...... if you have a coolant problem you'd check the belts???
Wouldn't you tend to believe the sensor and simply verify the coolant flow
prior to questioning your instruments?

CM

"Jeff Morris" wrote in message
...
| Oh! The ENGINE temperature!
|
| Even if there is a cooling problem, the engine provides a significant
buffer, so
| if the fluctuation is fast, as in you actually see the needle move, its
probably
| a loose connection on the sensor, or some other such fault.
|
| If its a small engine with a large alternator it could be a slipping belt.
I've
| never seen a case where the coolant flow went up and down repeatedly but I
| suppose it could happen. Can a thermostat fail in this mode?