I have a 1995 200hp EFI Mercury. I run the engine about 10 times per year in the salt water in Louisiana. The boat is trailered and stored under a cover. The engine runs flawlessly. The problem that I am having is rust on the engine under the cowling. The spark plugs on the left side of the engine (facing rear of motor) have rusted. The lower sprark plug is the worst getting progressively worse up to the top. When I remove the plug the electrode looks fine, just the outer part of the plug that you use to screw it in is rusted. With the cowling off and the motor running at idle on the garden hose, I can see no water coming out out of anywhere. I have not revved the engine for fear of lack of cooling on the hose. I am thinking maybe head gasket or something, but wouldn't that leak at idle? As I said before the engine runs great. There is also some rust forming on some of the valve cover bolts and some of the other metal parts below the left valve cover. Any suggestion would be much appreciated. Can the engine run great with a blown gaset? And if so is it damaged nonetheless? dave
Hi DaveThough i am not familiar with your exact type of motor, i think i can give you a possible hint.Some mercurys have a tencency of gettning small leaks at waterjacket around sparkplugs. Try to look right after you run the motor in water, at wot where water pressure is at highest. If there are small drops around spark plugs, you will need a new gasket and sealing there.