I have a 1986 mercruiser 140.
I have searched for a couple hours on rust on spark plugs. What I've found is that it's an indicator of a bad exhaust manifold and/or Riser, among other things. However, I have not found where this rust has to be on the spark plug to be a problem. Mine is on the outside.
I ran the boat today for the first time since November. After it ran for about 5minutes, I shut it off because my brass drain plug off the exhaust manifold was dripping. It wasn't completely in. When I pulled it off, all the water obviously came out, but I noticed these little black flake sediments. This caught me off guard.
A little while later, I went to check out the spark plugs and found this on 2 of the 4 spark plugs. There is also rust on the outside of the engine where these spark plugs screw into. It's like a bowl and I wondered if moisture was collecting there, causing the rust.
This boat did sit on a trailer all winter, under a boat cover, but the motor box was in the house. The boat cover is not 100% waterproof and the rain would sometimes get in the boat, and periodically, I would pump it out of the bilge.
My internet research on sediment in the exhaust manifold, coupled with the rust on the plugs, is starting to concern me.
My question is: Is this the classic "bad exhaust manifold/blown head gasket/Water getting in the engine" scenario or does the spark side of the plug look normal, and the rust on the outside just coming from being layed up not completely protected?
Thanks. I appreciate your help.