1996 7.4 Mercruiser 2 spark plug holes bridged over

I was winterizing my 7.4, and 2 of the 8 plugs had a sandy/powdery yellowish dry substance on them, it was the back last 2 plugs on the port side. When I looked in the holes, said substance is completely bridged over the holes. A few years back, I had a shady mechanic, who after doing a tune up, I mysteriously had water in the oil, it was foaming everywhere, he, at the time said I needed a new motor, but I refused, got the boat back, did a flush, changed the oil twice, replaced plugs, etc. The oil looks great. Could this be some sort of residue from way back? The past couple years the boat did have issues at higher rpms, but we mostly prefer to cruise at slower speeds anyhow, and this is apparently the smoking gun. I also put in a marine thermostat, the mechanic had installed a high performance one. My boat uses raw fresh water for cooling. Would it be safe for me to use a tiny attachment on my shop vac and suck this stuff out, then fog the engine real good, maybe squirt some mystery oil in those 2 cylinders, replace all the plugs and cross my fingers? Any advice would be greatly appreciated!! Happy Holidays

Frist off, you would have been better off posting this in the MERCRUISER section...

"...bridged over the holes..." - do you mean inside the combustion chambers?

What temperature rating was the 'high performance' t-stat rated at?

When was the last time the exhaust elbows were changed?

makomark said:

Frist off, you would have been better off posting this in the MERCRUISER section...

"...bridged over the holes..." - do you mean inside the combustion chambers?

What temperature rating was the 'high performance' t-stat rated at?

When was the last time the exhaust elbows were changed?

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I'm new here, but thought I was in the MerCruiser section, but thanks for the reply. I believe the high performance thermostat was like 185/190, and then I learned the 7,4 take like a 160, which is in it now. I assume that;s why it would go into more like an idle only "safe mode". And yes, there is a powdery substance bridged over where the spark plug electrode would be screwed into. It was only in the 2 Cylinders, the rest all are fine.
And I have had this boat for like 6 years, am unsure if or when the exhaust elbows were changed.

Leaking head gasket between the 2 adjacent cylinders? What does a compression test show?

fitz73222 said:

Leaking head gasket between the 2 adjacent cylinders? What does a compression test show?

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Thanks, have not done a compression check yet, I would have to research how to do that, I know it is not rocket science, but a previous mechanic I recall saying that is typically a waste of time, I don't know. This is my boat and was trying to save some money. But, makes sense that scale would build up over time if just a very small head gasket leak. The oil is fine, not milky or anything, and this motor has never used any oil at all before either

You're just looking for the static difference between a known "good cylinder" that is 160 psi and one of your suspect cylinders that is say 75 psi because of the gasket leak, "There's your sign"!