Mercruiser Woes-What Next?

[Bushwacker]

It sounds like you need new exhaust manifolds anyway, even if you buy a new long block, so I wouldn't sweat that! A compression test will tell you if you have any sticking valves or rings without even running it, so I'd do that first, before even buying new manifolds. If a cylinder is down in compression, squirt some oil in that cylinder and test again. If compression comes up it's a stuck ring or scored cylinder, indicating major overhaul required (or new short block!). If compression doesn't improve with oil in cylinder, it's a sticking or leaky valve or possibly a blown head gasket, especially if 2 adjacent cylinders are low. Requires pulling heads and having a valve job done.

If you got water in the oil, best thing to do is change the oil & filter and get it running asap and get it up to temperature for awhile to evaporate any remaining moisture. I would be a little concerned about rust on the cam if the engine got a lot of water in it, as the Chevy's use pretty small diameter lifters so there is a lot of stress on the cam lobe/lifter surfaces. If there is much distress there, the cam lobe will wear off pretty quick. If you can run the engine for a bit with fresh oil in it, get an oil analysis kit, which includes a syringe and tubing to pull an ounce or two of oil from dipstick tube, and have it checked for high levels of wear metals. This is a routine check on all aircraft engines. If wear levels are low, you're good to go. If levels are high, then you'll have to make a decision on how much work/time you want to invest on a rebuild or spend more $/less time on a new or rebuilt short or long block.

If you have a factory service manual, and decent mechanical skills, teardown and rebuild of a 1950's technology OHV auto engine really isn't that difficult, especially if it only involves pulling the heads or changing a cam & lifters. Could do either of those jobs with engine in boat. Toughest part of the job is probably pulling the rusted exhaust manifold bolts/studs! A read thru the service manual will give you a good feel for what's involved. Hope this input helps. Denny