Capt. JG wrote:
"hpeer" wrote in message
...
I have Volvo MD7A, 13 hp two cylinder unit with salt water cooling.

Runs perfectly with one exception, it occasionally overheats and I can't
figure out what the heck is going on. I bought the boat used and the
previous owner had fitter her out himself. Every time she overheats, I
find something to "fix" after which she is fine for a spell then she will
overheat again. Clearly I'm not getting to the root cause. Generally she
will seem to be running along just fine and all of a sudden the pressure
gauge just goes up, up, up. It seems to be a bi-state kind of thing, it
either works perfectly well or not at all. Its almost as if there is a
ball valve or something somewhere that is sticking. It's obviously not a
bad gage.

The plumbing goes like this:
Through hull fitting
Salt water strainer
Tap to the transmission/engine
Tap to secondary pick up from bilge
Tap to sinks


Here is some of the history and what I had done, in each case I have also
checked/changed the impeller. They usually look just perfect.

1: Recently fitted salt water strainer seemed to be blocked with debris
from its construction. It is home built out of some interlocking pieces
of PVC pipe, the inner piece has a zillion small holes drilled in it. I
dumped this stuff out and she was fine.

2: Found a small leak in the copper pipe from the transmission to the
impeller. I thought that maybe it was sucking in air and the impeller was
getting air bound. Repaired the leak.

3: Moved and lowered the salt water strainer. This was mounted pretty
high on a bulkhead with real long hoses. I moved it to the engine
compartment and lowered it. I thought that maybe there was just too much
suction head to get the water going.

4: Threw thermostat overboard.

Any clues?

Many thanks,

Howard


Do you have water flow out the back when it overheats? Is it normal flow?

Also, I'm thinking about a failing raw water pump... not the impeller. It's
driven by a belt... is the belt snug?


No, the engine is not sucking up water. The bilge intake is used when
winterizing so that I can put anti-freeze into the engine. The last
time this issue/problem occurred was during layup and I was not able to
get the engine to suck up anti-freeze.

This particular water pump is on a little shaft on the back of the
engine. I suppose the pump could be failing to turn intermittently, I
had not considered that possibility. Seems kinda far fetched, but then
again this whole problem feels weird.

"hpeer" wrote in message
...
Capt. JG wrote:
"hpeer" wrote in message
...
I have Volvo MD7A, 13 hp two cylinder unit with salt water cooling.

Runs perfectly with one exception, it occasionally overheats and I can't
figure out what the heck is going on. I bought the boat used and the
previous owner had fitter her out himself. Every time she overheats, I
find something to "fix" after which she is fine for a spell then she
will overheat again. Clearly I'm not getting to the root cause.
Generally she will seem to be running along just fine and all of a
sudden the pressure gauge just goes up, up, up. It seems to be a
bi-state kind of thing, it either works perfectly well or not at all.
Its almost as if there is a ball valve or something somewhere that is
sticking. It's obviously not a bad gage.

The plumbing goes like this:
Through hull fitting
Salt water strainer
Tap to the transmission/engine
Tap to secondary pick up from bilge
Tap to sinks


Here is some of the history and what I had done, in each case I have
also checked/changed the impeller. They usually look just perfect.

1: Recently fitted salt water strainer seemed to be blocked with debris
from its construction. It is home built out of some interlocking pieces
of PVC pipe, the inner piece has a zillion small holes drilled in it. I
dumped this stuff out and she was fine.

2: Found a small leak in the copper pipe from the transmission to the
impeller. I thought that maybe it was sucking in air and the impeller
was getting air bound. Repaired the leak.

3: Moved and lowered the salt water strainer. This was mounted pretty
high on a bulkhead with real long hoses. I moved it to the engine
compartment and lowered it. I thought that maybe there was just too
much suction head to get the water going.

4: Threw thermostat overboard.

Any clues?

Many thanks,

Howard


Do you have water flow out the back when it overheats? Is it normal flow?

Also, I'm thinking about a failing raw water pump... not the impeller.
It's driven by a belt... is the belt snug?


No, the engine is not sucking up water. The bilge intake is used when
winterizing so that I can put anti-freeze into the engine. The last time
this issue/problem occurred was during layup and I was not able to get the
engine to suck up anti-freeze.

This particular water pump is on a little shaft on the back of the engine.
I suppose the pump could be failing to turn intermittently, I had not
considered that possibility. Seems kinda far fetched, but then again this
whole problem feels weird.


Seems like something that intermittent wouldn't be related to restricted
flow, since if it's restricted, it's not going to get better.



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

"hpeer" wrote in message
...
This particular water pump is on a little shaft on the back of the
engine. I suppose the pump could be failing to turn intermittently, I had
not considered that possibility. Seems kinda far fetched, but then again
this whole problem feels weird.

I have had this trouble on a petrol engine in my 22' motorboat and the cause
lay in the rubber impeller in the pump.
It is easy to examine the impeller and, seeing no wear on the ends of the
lobes, to conclude that it is Ok for further service.
However after some time the rubber goes soft and this means that the lobes
cannot spring back quick enough after passing the cam to allow water to come
in through the inlet.
You may get away with a soft impeller at low speeds but when you increase
revs you will lose suction..
Also, with a soft impeller the pump becomes more sensitive to back pressure,
which in extreme cases can allow back leakage past the ends of the lobes.
Your system seems likely to create quite a bit of back pressure especially
if the taps you have in the system are not gate valves or ball valves which
allow full bore passage through them.
You need this sorted out quickly because you are salt water cooled and every
time you get temperatures over 140 degrees deposits will come out of
solution and ultimately cause severe problems.

In message , Edgar
writes

"hpeer" wrote in message
m...
This particular water pump is on a little shaft on the back of the
engine. I suppose the pump could be failing to turn intermittently, I had
not considered that possibility. Seems kinda far fetched, but then again
this whole problem feels weird.

I have had this trouble on a petrol engine in my 22' motorboat and the cause
lay in the rubber impeller in the pump.
It is easy to examine the impeller and, seeing no wear on the ends of the
lobes, to conclude that it is Ok for further service.
However after some time the rubber goes soft and this means that the lobes
cannot spring back quick enough after passing the cam to allow water to come
in through the inlet.
You may get away with a soft impeller at low speeds but when you increase
revs you will lose suction..
Also, with a soft impeller the pump becomes more sensitive to back pressure,
which in extreme cases can allow back leakage past the ends of the lobes.
Your system seems likely to create quite a bit of back pressure especially
if the taps you have in the system are not gate valves or ball valves which
allow full bore passage through them.
You need this sorted out quickly because you are salt water cooled and every
time you get temperatures over 140 degrees deposits will come out of
solution and ultimately cause severe problems.



Another problem I have had with impellers is - they look OK, but the
central bronze part becomes un-bonded from the rubber, and starts to
spin without turning the rubber. This might be intermittent by having
different friction at different speeds.

--
Spike