Boat Maintenance & Repair Forum

Latest issue / ongoing problem

by BruceK » 30 Nov 2016, 17:30

This is why I went for blow by as a first stab rather than head gasket. If your head gasket is gone I'd suspect emulsion in the sump too. But with blowby the water is in a gas state and condenses in the coldest part of the engine first. This invariably is the rocker cover and particularly the filler cap. However the rings may not be completely shot. As with most marinised engines with a cast iron head and particularly with raw water cooled engines, the tendency is for engines to run colder than initial design. This will allow blowback which is why your typical wife's car if she is a traditional house wife, always seems to have mayonnaise under the filler cap. It's a symptom of running the engine cold and never really getting it to temp (if the school run and shops are just round the corner that is). Your's however is significant by it's quantity. Either you are perpetually running your engine cold or you have stuck / worn rings. And this can all be exacerbated by water in petrol tank / humid environments or wet bilges or poorly positioned air intake vents in the hull etc

That's my initial diagnosis anyway. Thereafter I would look at head gasket and head installation procedures to see if you missed anything. Finally I would look at any oil heat exchangers that use raw water, and by implication any o-rings etc that seal the engine internals from any coolant. Lastly I would look for block cracks and by extension manifolds etc.





When you decouple your crank case breather are you getting any significant blow by? If it goes to one rocker cover only is that the one that has more or less emulsion?
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by RPM Marine Services » 30 Nov 2016, 20:11

Hi Shibbs,
When you took the plugs out to carry out a compression check, was there any water in the cylinders? Also when were the exhaust manifolds last changed?
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by shibbs » 30 Nov 2016, 21:03

Manifolds are about 5 years old but have only had about 2 years worth of use. Boat wasn't used much at all by previous owner. I've done 2 full seasons on them.
There wasn't any water in the cylinders, plugs looked fine and I turned it over with one plug out at a time with a cloth in front of the plug hole to see if any water came out.. nothing.
Stu

Draco 2500 Crystal - Mercruiser 5.7, Alpha one - Chin Chin
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