In message , hpeer
writes
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?

You've had many replies - and good clues to look for.

Whilst at it, I would look at your flexible exhaust - if this has
laminated and partially blocked the exit it will cause overheating. And
it likely has laminated if you've overheated..
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Keith replace nospam with ilf0rd