Water on sparkplug

"I have a 120 HP Mercruiser, v

"I have a 120 HP Mercruiser, vintage 1975. It was running poorly so I went to do a tuneup. It had sat unused for a week before I did the tuneup. The first three spark plugs were nice, new, clean, perfect. The fourth (rear) plug had slimey goop up inside of it and a drop of clear liquid water (not gas) on its tip.

A friend suggested head gasket. I rejected that because the plugs are above the head gasket so I don't see how water could drip onto them.

Another friend suggested exhaust manifold or elbow. That seems more likely to me because they are above the plugs.

A cracked cylinder head was also considered, but we don't want to think about that. :-(

Any advice or suggestions? How are are these parts to find and to replace?

I am a reasonably good mechanic, but not a pro and this is my first Mercruiser, so feel free to treat me as a newbie."

The first order of business is

The first order of business is a compression / leakdown test.

This will answer your question.

Parts are readily available and the gaskets are not expensive.

"Ayuh,...

The Riser/ Elbow

"Ayuh,...

The Riser/ Elbow can be checked by just pulling it Off,+ looking for Rust Tracks....

The Headgasket can be checked by Compression,+ Leak-down Tests....
A Cracked head might possibly show up thru a Leak-down test..."

"Thanks. At least you guys di

"Thanks. At least you guys didn't add to the list.

My engine tools were stolen a few years ago and I have not replaced the compression tester. How do I do a leakdown test? If that's cheaper, maybe I'll do it first."

"Oh, and I'm going down ri

"Oh, and I'm going down right now to pull off the exhaust elbow and look it over. Thanks."

"Ayuh,...
I hook my air hose

"Ayuh,...
I hook my air hose to my compression tester's adapter hose,+ pressurize the cylinder...
'course, you have to make sure the motor can't turn over...

It's alittle less scientific than a Gauge,...
But I can still Hear where it's leakin'..."

Spend the $20.00 and buy a new

Spend the $20.00 and buy a new one. You're going to be getting your wallet out anyway and it's the cheapest thing you will need for this repair plus you'll be getting accurate information we can use for comparison.

Just thought everyone would li

Just thought everyone would like to know: I took the exhaust elbow off and looked inside. It was rotted out. She runs fine with a new exhaust elbow and the oil is remaining foam-free.