When you say my head gaskets blew..........are you saying both blew at the same time?
They may have, its hard to know for sure. What happened was I had a bad overheat back in '7/2013. Pulled the boat out and replaced the melted flappers, exhaust hoses were not burned but changed them anyway. Did a compression test and results were normal believe it or not. Ran it the rest of '13, 14 and '15. It ran fine, no odd noises...then at the end of August '16 it did not want to start. When it did start it ran rough for the first few minutes. I'm thinking tune up or fuel. I look at all the possibilities but nothing really comes up. Then for the heck of it, I check for water in the cyls, and water shoots out of #2. I looked at all the plugs and #2 and #1's center electrodes were orange, so I knew I had water in #1 also but not enough for it to spray out. I drained the block and had it towed in. Did some more diagnostics in the driveway, believe it or not, comp test results were basically normal. 145-175. I rigged up a way to see if combustion gases were getting in the cooling water and for sure they were. And by then the oil was getting milky. I changed the oil 2x, and drained the block. Fogged it really well.
Got a warm spell in Feb and started taking it apart. Ironically the heads came off easy with the help of my De Walt impact gun, it was the intake that gave me fits, the old gaskets were baked on like Kryptonite!
The head gaskets were blown in both #1 and #2, there was a slit in the fire rings that you could easily see. So I guess that the overheat weakened them but they did not fail to the point of letting in water till the end of last summer. I finally got it all back together with a pair of re-man heads, Fel Pro gaskets and a new Volvo style center riser exhaust replacing the batwings. Ran it on the water hose today and it sounded great.
PS fogging it several times then cranking it with ig disabled and plugs out and then fogging it again thru the plug holes really helped. This engine had salt water in it for probably 4/5 days, and when I pulled it apart in Feb after all that fogging, there was NO rust in the cyls at all.
PPS I know people scoff at the Mercruiser oil being NMMA rated, what does it really mean, is the extra corrosion resistance really significant. Well after I took it apart down to checking each lifter for corrosion....there was....NONE....not on the lifters, not in the lifter bores, not on anything but a light bit of rust on one of the pushrods. I used this oil probably for the past 5 years and I'm thinking it may have helped, who knows. But I would have expected an engine that had salt water in it, would have had significant corrosion.....
