Well, we had a problem with the intake being blocked and
the impeller not functioning well. That caused temp fluctuation
as you described.

--
"j" ganz @@
www.sailnow.com

"Bart Senior" wrote in message
. ..
1 point for you Jeff.

The correct answer is Coolant level. When the coolant is low
it can steam and vary the sensor reading.

I had a problem with overheating on a freinds boat on Wednesday.
I was on a tight schedule to get some kids to a class they were taking
and had to rely on the engine to meet schedule, but didn't quite make
it..

As I reconstructed the problem, the strainer which consisted of a
metal basket with its strap hoop handle was installed improperly. My
guess is the stainless strap partially blocked the raw water intake. It
worked fine until the engine was rev'ed up a bit where upon the flow
was not satisfactory. In and out of the slip under power would not
reveal the problem.

Anyway, the engine overheated and thrashed the impeller, or perhaps
vice versa. The coolant boiled off and needed to be replaced.

The fix was to bend the strainer strap into an "M" shape, because I
could not tighten it enough to close the cap--well perhaps I could have,
but I was worried about breaking the studs holding it down. That
would have meant a new strainer.

The impeller installed fine until I had to install the circlip which
sprung off and disappeared--I never found it. A friend ran one
up to me. I found all the impeller pieces except one which may
turn up in the heat exchanger. I'll let the owner deal with that
one.

Jeff, your impeller problem is weird. I didn't think the impeller
could spin on it's shaft. Didn't it have a keyed bushing of some
sort?

Bart

Jeff Morris wrote

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

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?


"Jeff Morris" wrote

| 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?