hpeer wrote:
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
With that history, the impeller is almost certainly junk. Replace it
every three years minimum or after *any* serious overheat.

If you've got a delaminated hose, its interior may be collapsing under
suction with nothing visible on the outside, blocking all flow. It will
spring back when the engine is off, either immediately or when
disturbed. With an intermittant problem like this, replace *all* hoses
between the intake and the water pump if there is *any* sign of
distortion or they are over about 5 years old with suction rated hose.
The collapse would be triggered by a momentary obstruction e.g. a
plastic bag or clump of seaweed over the intake which washes clear as
soon as the hose collapses.

On many engines removing the thermostat *will* result in cooling
problems. Any bypass type thermostat *MUST* be in place for proper
cooling and even a direct flow thermostat (only two ports on its
chamber) can give trouble if missing. A seawater thermostat is easy
enough to test in a pan of water on the stove so deep sixing it was not
so smart. In an emergency, for a direct flow system, snipping out the
central plunger will let you make a long motor passage to reach port
with a failed thermostat, but if totally removed you should consider it
harbour use only. Some engines will run happily with no thermostat but
it all depends on the design of the water jacket and should *never* be
counted on). For a bypass thermostat (3 port chamber) in an emergency
either wedge it wide open, or you have to plug the bypass hoze.
I belive your thermostat housing has a 1/4 inch bypass port and I have
heard that it also doesn't seal properly without a thermostat in.

It doesn't matter if the strainer is below or slightly above the water
line so long as its absolutely air tight, and there is more water volume
in the hose to the water pump than the volume of the strainer + the
volume of that part of the intake above the waterline. Its going to
have a bubble anyway and if you really want to be able to quickly
visually check the flow in a large clear lidded strainer put a pingpong
ball in there, ideally with half of it coloured black in sectors to make
it easier to see it move. Personally I'd prefer below the waterline,
with a nylon or marelon pipe coupler to join its intake and outlet hoses
cable-tied to it for emergency repairs. The standup bronze one on the
seacock, you can just shove a tapered bung into which should be there
anyway.

No air leaks or restrictions other than the strainer can be tolerated at
all between the intake and the pump. You should remove the tap to the
bilge and the galley and connect direct to the seacock. If the problem
is solved, I *might* consider reconnecting the bilge pickup, but the
amount of emergency seawater pumping this gives you is minimal and a
large mechanically or electrically driven pump would be a far better
idea. You can even get a dewatering pump that permanently clamps round
your propshaft and acts as a bilge blower as long as the water is below
its housing. Google 'Ericson Safety Pump'. Unless one has a seawater
manifold with an intake of at least 3 times the total area of *all*
pipes off it, it is an extremely bad idea to tee *anything* off the
engine cooling intake. The additional risk due to a properly installed
and maintained seacock for the galley in a protected location is
negligible, and it can be easily and properly fitted in a couple of hour
yourself for less than a affordable meal out for two.

If you are concerned about cooling flow, one of the simplest ways of
monitoring it is to run a small bore pipe from the vent spigot on a
non-valved vented loop between the engine and the injection elbow to a
little telltale just over one of the cockpit drains, NOT an external
through hull, so you can see there is adequate cooling at any time.

If, as I do, you have a bronze strainer with wing nuts (actually wing
bolts on mine), keep some spares. The one time you are under pressure to
clear it, you'll loose a nut down the bildge. . . :-(

You've probably got a partially blocked water jacket on the manifold or
block. It will require acid flushing. I've had good results with a
formic acid based kettle descaler which is less aggressive than the
hydrochloric acid based products so safer for pumps, thermostats etc.
There are various methods of flushing ranging from a static soak
followed by hoseing out to running the engine while circulating the acid
solution, best ask here for further advice. A dilute Phosphoric acid
(Jenolite or milk stone remover) flush after you've got the rust and
lime out will convert remaining rust to Iron Phosphate, reducing the
risk of further rusting. Volvo used to recommend inhibiting the cooling
system with a soluable oil when laying up.

P.S. there is an engine workshop manual he
http://www.bluemoment.com/manuals/Vo...A_Workshop.pdf