Mercruiser 5.7 Alpha 1 overheat

[ukstu]

Hi All,

I'd appreciate some of your advice if possible please. I have a 1991 Mercruiser Alpha 1 with approx 700 hours on that is used on salt water in the UK.
I went out today and about half hour into our journey it overheated. I shut the engine down and awaited a tow back in.
After about 2 hours, and back alongside, the engine temp was reading back below running temp so i took the inlet hose off the thermostat connection and turned the engine over to see if i had water pumping in. There was, although whether it was enough i am unsure.
Due to this, i fired the engine up again and ran it to see what the temperature did.
All hoses that should be cool were and both exhaust elbows were cool to touch so this told me that water was circulating to a certain point.
The temperature did start to rise again though.
Engine was shut down and the questions are now being asked...
The engine is well looked after, regularly used etc. Full service including new impeller took place at the start of the season ( i know this doesn't necessarily mean that the impeller wouldn't have failed).
As the boat is kept in the water, and stripping the drive leg down would be my last choice without chasing all options down first, could anyone please advise on things i can check / any ways to narrow down where the fault is without having to pull the boat out?

many thanks in advance, ( sorry for the long story but trying to give as much info)
Also, is there any way of measuring how much water should be getting pumped in from the impeller ( i.e from where i can split it? )

Thanks all...

 

[Bondo]

that is used on salt water the boat is kept in the water,

Ayuh,..... How long ago were the exhaust manifolds, 'n elbows serviced,..??

The water gettin' in,..... has gotta get Out,....

 

[ukstu]

Manifolds and elbows were off during the previous winter and all in good condition..

 

[ukstu]

Also, when i re-started, they all remained cool to touch..? Does that mean the water is passing through correctly? Was thinking, if i disconnected the elbows from the Y joint i could run to see if water is pumping? I plan to disconnect anyway to check flapper conditions.

 

[Bondo]

Also, is there any way of measuring how much water should be getting pumped in from the impeller ( i.e from where i can split it? )

Ayuh,.... Take the hose that comes from the raw water pump, off where it hooks to the t-stat housin',.....

At idle, the solid stream of water should bubble straight up, 'n topple over on itself, 4, or 5" from the hose,......

 

[Scott Danforth]

or time it, you should fill a 20 liter bucket in about 20 seconds

 

[ukstu]

so, did a bit of investigation today. Impeller is absolutely fine, pumping more than enough water.
I then decided to do a compression test. left side ( as you look at it was absolutely fine, new head at start of year), right side, numbers seemed low but all ranging from 120 - 145, ( 1 - 120, 3 - 120, 5 - 135, 7 - 145) i thought this not too bad considering quite an old head that side.
however, at this point i realized that i had water in the cylinders of 1 and 3.
Initially i assumed this would be down to the manifolds, but, taking the manifolds off and turning the engine over ( plugs out) it continued to pump out water from the plug holes. Without the manifolds there the water must be coming from the head..?
Now i'm assuming this is either a knackered head or a blown head gasket.
My confusion now, is, if the impeller is pumping, ie no cause for overheat, what caused the over heat, if it was a blown head gasket , what caused the head gasket to blow. If it is a knackered head, then i assume the water ingress will cause high temp?

Please let me know if my thinking is on track...

either way, i know i'm pulling the head off again to see whats going on.

Frustrating part is, both heads were off at the start of the season due to milky oil ( one corroded head replaced.. maybe i should have replaced both, but funds are not always there)

As always, thanks for any advice / input.

 

[Scott Danforth]

pull the boat and get it on the hard

drain the block and heads

do a water jacket pressure test with air (should hold 15 psi forever)

if its not holding pressure, follow the hissing

BTW, 10% is the maximum difference between high and low cylinders. you have much more than 20% indicating something is amiss