I'm fairly certain of manifold deterioration but I just wanted to get a second opinion on this issue from others who know more about boat engines than myself. I had an issue on the water recently where the engine was hesitant and RPMs were bouncing around intermittently. Today I finally got around to doing a compression test and replacing the plugs. All 3 cylinders on the starboard side were fine. When I got to the second side, all 3 of the plugs were rusted in tightly and one of them I couldn't even get out. The 2 cylinders that I did remove the plug from had good compression, but the plugs were moderately rusted and corroded all the way to the electrode and appeared to have water on them. The cylinders were spitting out a dirty water residue as well when cranking. The engine is a 1990 and I believe the manifolds are original. It was infrequently used in freshwater until I purchased it 3 years ago and it has been saltwater & far more frequently driven since then as well. I do not know all that much about boat engines, but I feel fairly certain the manifold has hit its end and I was just hoping for some potential confirmation from someone who knows more than me. The only other way I could think water would be able to get into the cylinder would be through the head gasket and with the compression being proper on the cylinders that showed water I don't suspect that is the problem. I would appreciate any thoughts. TIA.
