On Sat, 19 Jun 2004 09:07:01 GMT, "Barleycorn"
wrote:


Hi Barleycorn

The head on this engine is cast iron and doesn't warp very easily, but
even if the engine pump fails, you still have quite a bit of
circulation from the leg pump.

Did the engine actually overheat? if not, its likely little damage was
done unless you were really pushing it.

The engine is the old General Motors unit used in American pick-ups et
al, and there are quite a few spares available, but the only heads I
have replaced were due to poor maintenance where valves and head
gaskets have failed.

Suggest you carry out a compression test before going to the expense
of replacement parts and let us know the results, plus any other
details that made you consider a failed head.

HTH

John

Hello John,

Thanks for your help. When I started the boat for the first time in the
spring I noticed that it was 'breathing' heavily, especially on tick over.
When I took her out last week the engine ran very rough, on three cylinders
a lot of the time. Also I noticed the temperature gauge going right up to
the red, and then coming down to normal, every couple of minutes. I put
this down to the thermostat playing up perhaps. Also she was a bitch to
start when hot. We usually cruise at about 3000 revs when out, too tight to
buy the petrol! When I got her home and put the bellows on to wash the
engine through I noticed a rattle in the water pump. I pulled the plug leads
off, and found that the cylinder farthest away from the water pump was not
firing. (Swapped sparking plugs, but still same cylinder). I did a
compression test, and cold this cylinder was 130, while the other 3 were
150. After a squirt of oil in the bores this one went up to 150, while the
others went to 170. Also the oil has gone milky. From all this I assumed
the head gasket may be the problem. I have removed the head, but the gasket
was perfect. However from the rust in the head / valves on the poor
cylinder it shows that it has not been running. The bore is a little
marked, I did wonder about stuck / broken piston rings? On removal of the
water pump I found that the impeller has worked loose on the shaft.
The reason that I suspected the head was that I imagined that perhaps the
farthest end of the head was starved of water due to the engine pumps
failure, and created a 'hot spot', perhaps cracking it, thus letting water
in the oil, and causing the poor compression.
Having said all that, I am more used to diesel engines in tractors, so I am
really grasping in the dark. I have found a firm locally who will pressure
test the head, and can treat it with some sort of Teflon coating if it
leaks, but how this would stand up to sea water I am not sure. They do not
like welding cast iron. They charge £45 for the test, so I am just going to
improvise with a metal plate, some bungs, and an airline to see if I can
find a leak first.
If I am in trouble, I have found a 2 1/2 litre Mercruiser engine with a new
head for sale. Do you think that they are interchangeable?

Many thanks again for your help,

Barleycorn.

Hi again barleycorn,

OK so you seem to have done the 'good' things, lets take it one step
at a time.

When you laid up the boat at the end of the last season did you oil
fog the combustion chambers? failure to do this can result in stuck
rings and lead to excessive breathing from the crancase vent, it will
also give the low compression reading ( although not that bad at cold,
a good pressure warm with throttle wide open is usually about 150
psi).

I assume the milky oil is in the cylinder being tested and not the
sump, either way water is definately getting into that particular
cylinder. As its No4 cyl. which sits lowest in the boat during normal
running, it is possible that due not firing the cylinder is causing a
partial vacuum within the exhaust manifold and sucking water back from
the discharge and into the cylinder.

Too much hose pressure while flushing with the muffs can cause the
same thing to happen.

I have managed to get home with broken pump belt with just the
pressure from the leg pump with no damage, but I was being careful.

It is highly possible that if the cooling system wasn't adequately
inhibited upon laying the boat up severe corrosion can damage the head
and exhaust manifold especially if used in salt water conditions. you
can check the manifold off the head with some home made bungs and a
hose pipe, water should only be discharged from the overboard/exhaust
outlet, if any comes out of the head/ manifold ports there is a breach
within the manifold.

This is where I would put my money on a crack or corrosion hole
between the water way and inlet valve porton the manifold of No4
cylinder.

The head and block can only really be checked properly by leak
testing.

I'm not sure about the 2.5 head, never had one, but if it is the same
you could have problems with compression ratios

Look hard at the manifold.

John