In article ,
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.

Any clues?


My Yanmar started overheating (according to the gauge anyway) and after
stripping it all down, I finally found a tiny hole in the raw water
jacket into the exhaust manifold.

Does your fresh water level go down when it overheats?

--
Molesworth

"IanM" wrote in message
..
snipped.
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.
more snippage

I think using the engine pump on bilge water is a very bad idea, not just ,
as you say, that the quantity pumped is minimal, but also because all sorts
of crud lie in bilges and do you really want to pump such 'stuff' through
you engine? Maybe you will create a blockage that will be a real PITA to
clear.
Also, unless you are in a position to sit there watching and turn back the
cock to sea water inlet at the precise moment before the bilge suction sucks
air you are going to send a lot of air into your engine which will create
local overheating and maybe an air lock or even a steam lock and maybe
crack the cylinder head when the cold water finally comes through.
I agree with your suggestion of an entirely seperate bilge pump of adequate
capacity.

Nigel Molesworth wrote:
In article ,
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.

Any clues?


My Yanmar started overheating (according to the gauge anyway) and after
stripping it all down, I finally found a tiny hole in the raw water
jacket into the exhaust manifold.

Does your fresh water level go down when it overheats?

Raw water cooling so, no.

Edgar wrote:
"IanM" wrote in message
..
snipped.
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.
more snippage

I think using the engine pump on bilge water is a very bad idea, not just ,
as you say, that the quantity pumped is minimal, but also because all sorts
of crud lie in bilges and do you really want to pump such 'stuff' through
you engine? Maybe you will create a blockage that will be a real PITA to
clear.
Also, unless you are in a position to sit there watching and turn back the
cock to sea water inlet at the precise moment before the bilge suction sucks
air you are going to send a lot of air into your engine which will create
local overheating and maybe an air lock or even a steam lock and maybe
crack the cylinder head when the cold water finally comes through.
I agree with your suggestion of an entirely seperate bilge pump of adequate
capacity.


Agreed on all points about the bilge pickup being used as a bilge pickup.

I ONLY ever use it as a way to put in anti-freeze during layup.

IanM wrote:
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

Thanks for the long and detailed response. Many good points to ponder
and things to sort out. Adding the extra thru hull may be something I
need to do but I hate the idea of it. And also, it is a steel boat.

The thing that drives me crazy is that it is just so darn intermittent.
It can run for hours and hours (day and days even) perfectly and then,
in an instant, the temp goes up. You can see it rise.

I've gotten to the point of dreading to run the engine for I now end up
constantly staring at the temp gage. The aggravation takes out all joy.

I have an MD7A vintage 1977 with salt water cooling and began to get white smoke and overheating at RPM over 1000. Problem ultimately traced to partially clogged input to water pump. Removed right angle pipe fitting on starboard side of transmission and removed crud from inside transmission while blowing water through from water hose into port side of transmission. You should be able to pour water thru here freely. This solved the problem so that without thermosat in place the temperature never went off the bottom of the gage. Then replaced thermostat and got steady 140 degree temperature.

I have an MD7A vintage 1977 with salt water cooling and began to get white smoke and overheating at RPM over 1000. Problem ultimately traced to partially clogged input to water pump. Removed right angle pipe fitting on starboard side of transmission and removed crud from inside transmission while blowing water through from water hose into port side of transmission. You should be able to pour water thru here freely. This solved the problem so that without thermosat in place the temperature never went off the bottom of the gage. Then replaced thermostat and got steady 140 degree temperature.