If you were my customer and I conducted a pressure test of the cooling system and found no leakdown, ran the engine and found no above normal increase in pressure of cooling system while running, saw no steam indicating a blown head gasket
, and had good compression
on all cylinders. I would adopt a wait and see attitude.
It is at this point where some people/"mechanics" start throwing parts
at engines to try to fix things...that costs you a lot of "boat bucks"
As far as the Oil Analysis....How were the other readings for metals/salt/etc.
The ananlyses that I use give acceptable levels and abnormal levels.
One oil ananlysis gives you a benchmark...you may have to change the oil and run the engine for a while and re-test.
If the engine is loosing coolant
after shutdown what may be happening is the water
is migrating along the headgasketfrom the cooling passeges into the cylinder due to vacuum as the cylinder cools...this is assuming that the cylinder has no open valve.
If the engine is not overheating
, and the oil is not milky, and the exhaust
my advice is to observe/record the performance of the engine.
Just a final note....On Yanmars there are a couple of block drains...make sure they are not "weeping"