I have a 1999 Baja 252 with a 454. Last week i sucked mud and over heated the engine (did not drive hot), when I stopped to shut off the engine, the engine hydro-locked. Repairs where: back flushed engine and new impeller. This week, the engine over heated AGAIN (after running 1 hour fine), I shut off the engine, the engine hydro-locked AGAIN. How is water getting on top of the pistons?? Do you think it is coming through the exhaust? We ran a compression test and received a good test. Also, the water intake is at the lowest point, do you think a little dirt will cause the engine to over heat that easy? Thanks for your help!

water gets there all the usual ways... head gaskets, intake gaskets etc as well as a bunch of marine specific ways like cracked /corroded manifolds. and on mine the intake manifild had actually corroded away . what you have to do is the easy stuff first which is get the plugs out and get the cylinders oiled down and the oil out of the motor etc... that will at least preserve what you have until you figure out whats wrong. if it were mine i would have the intake off and the exhaust off . that lets you look around for obvious things and get the exhaust pressure tested. then the heads if all that is ok... just methodically work your way down. the answer will be self evident when you get to it. you can spend your time doing compression and leak downs but , in this case, i think since you are looking at substatial disassembly anyway you might just as well get at it. you will end up holding all the same parts in your hands anyway.

I had a very similar situation on a 272 baja with 454 mag. I purchased the boat from a local marina. The motor would run and then shut down, when trying to restart it would hydro lock.I pulled the pluge and emptyed water from cylinders, changed oil and reinstalled new plugs, It happened again. i was told that risers and manifolds were new. I pulled risers and found "under gasket" a rust spot that allowed water into cylinders. when confronted, the marina replaced with new at no cost to me. the bad part is by trying to start the motor i knocked teeth off of the fly wheel and had to pull motor to replace. also broke starter bolt off. the moral of the story is look under gaskets.

did the engine diesel when you shut it off ?? seen that happen when hot and sucks water right in...

Possibly overheated the engine and cracked the exhaust. Pressure test the exhaust.

Hit a plastic bag once and over heated the boat and it took the exhaust gaskets (and connection hoses) out.

Cracked a head or blew a head gasket from the original overheat.