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.