checked oil on motor looked new motor ran fine took the boat off trailer was by myself made a hard pass to 5800rpm ran at that rpm for a minute then it started loosing rpms. pulled back on the throttle sounded like it was loading up with fuel,raised the rpm to about 25-3000 rpm and it cleared up but still has a miss or lazy sound, went to plain out again went up fine the falling off again,clean it out again loaded it back up. pulled plugs looked good, one plug was loose in the hole lots of water in the oil think its a headgasket,maybe a piston?
the motors are 557ci with 871 , innercooled , less than 10hrs

No signs of water in the cylinders ? Only in the oil ? Did you try and crank the engine after having removed the plugs ? That will show if you have water because it will spit out of one or more cylinders ?

could it be the intake manifold?

i see you are in Ok.......... you didn't inadvertantly freeze this thing last winter or something.... seems odd to have it run perfect when you out it on the trailer the last time and now you take it off and its dead.... not saying you are wrong but that sort of defies logic...

i would be looking directly at what ever you did between the last time it ran good and now... thats where the problem is likely to be for whatever reason...

Intake manifold gasket is the most likely cause. Cylinder head gasket is not very likely. Other possibility would be a crack in the block or cylinder head. I would bet on the intake manifold. Don't run it anymore until you find the problem, it will cause bigger problems.

Rene

I think head gasket is as likely as the intake gasket especially since you ran hard right before the motor started acting up. Look on the side with the loose plug. A burnt piston won't necessarily put water in the oil pan. Mine detonated, pushed the water jacket portion of the gasket into the valley and filled the motor with water. Did exactly what you described with no other damage. Leakdown showed OK before tear down.

ok, we ran a leak down checked ok,all within reasonable reading, same as other motor,did a compression ck same on all cylinders. presure cked header leaking on #2 cyl :eek. the minute it acted up i put it on the trailer. maybe we dodge a bullet.

At least you caught it before anything bad happened. Since your compression and leakdown looked good, I wouldn't worry about the engine at all. If it were me, I would pull the headers and see if they could be repaired. Replacing those is kind of pricey. BTW, I'm not sure, but I think it's Trick Marine that has them on ebay (CMI Sport Tubes) and here. I wouldn't go to mercruiser for them.

I Was thinking a little more about this problem and am wondering if the headers would be a likely source for water in the engine. For water to find it's way into the crankcase, it would have to get by the pistons. That seems unlikely with good compression and leakdown. I think it would be a good idea to keep looking for another source. I would probably try pressure testing the cooling system (engine only) and get out a stethoscope. I believe (although I hope I'm wrong) that you still have an internal leak.

Hey Thunderstrucck, I wonder how common the failure of the gasket is the way it happened to yours? I hadn't even thought of it being a real possibility until I read that it happened to you. Hopefully I'll be able to remember it the day that I need to.

Good luck,

Rene