Possibilities include a hydrolock from water being 'sucked back' from the exhaust system with one (or two pistons - piston rings actually) frozen/rusted to the cylinder walls). This is 'reversable'. It is VERY IMPORTANT that you DO NOT FORCE the engine over, either by starter or by hand at this time .... or you can crack/break the piston rings.
1. SHUT OFF the FUEL SYSTEM at the tank shut off valve.
1a. Get a long handled 1/2" 'breaker bar' and appropriate metric socket. Place on the pulley bolt thats mounted to the crankshaft and GENTLY (DO NOT FORCE) attempt to SLOWLY rotate the engine clockwise. If the piston rings are frozen to the cylinder walls or the piston ring grooves are frozen and you force it, you WILL break the piston rings.
If the breaker bar freely and slowly turns the crankshaft, then look for a jammed starter with its 'bendix drive' still engaged into the flywheel - remove and repair the starter and the job is done.
2. IF the starter is free and the engine is still 'frozen': GENTLY attempt to turn the engine clockwise, you may attempt to rock the breaker bar back and forth ... but do this 'gently' and without much force.
3. If the engine wont turn: Immediately go to an automotive parts shop frequented by 'mechanics' (NAPA, etc.) and get some "Marvel Mystery Oil" - a quart will do.
4. Clean out / Vacuum out the injector bores to prevent any crud from entering the engine when you remove the injectors. remove the injector holddown bolts, the injector feed lines .... re-inspect for crud in the bore holes, then remove the injectors.
5. pour a few tablespoons of Marvel Mystery Oil down the Clean injector bore holes and into the combustion chambers.
6. Let soak for a few DAYS.
7. Attempt to gently rock the crankshaft bolt back and forth ... but with LOW pressure from the breaker bar. If it breaks free, then fine ... add more Marvel, turn the crankshaft TWO full turns, add a few tablespoons more of Marvel Mystery Oil and let soak for a day or so. If it doesnt break free, just keep adding MMO and let soak a day or so more, then repeat the 'rocking back and forth' adding MMO .... until it does break free. Patience and MMO eventually will get the piston rings free of rust.
8. After the piston rings 'break free' ..... add more MMO and 'with the breaker bar', rotate the crankshaft many times.... but do not force it if you feel a LOT of resistance.
9. Once you are SURE that there is no resistance in rotation, then add a bit more of MMO and crank the engine with the starter (dont overheat the starter).
10. Drain the engine oil and add a 50:50 mix of Marvel Mystery Oil and normal engine lube oil. Important - Suck out any MMO that you can from the combustion chambers. Reinstall the injectors, bleed the fuel system. and START the engine. (CAUTION: REmove the 'air cleaner' and its sheet metal container from the air intake of the engine. Have a thick rag standing-by in case the engine 'runs away' at INFINITE rpm - jam the balled-up RAG, not your hand!!!!! into the air intake to stall the engine). Start the engine and allow to run at IDLE SPEED ONLY. Let the engine slowly come up to operating temperature .... increase the engine speed to a 'high idle speed'. When at operating temperature (140-180° F) slowly increase the rpm to 1/2 cruise speed and keep the engine at this speed for an HOUR. It is VERY IMPORTANT that you DO NOT APPLY ANY LOAD to the engine with MMO in the crankcase ... no putting the engine in forward or reverse gear, etc. This will 're-polish' the cylinder walls, re-polish the piston ring grooves, etc.
10. Remove the engine oil/MMO mix and replace with engine oil.
To prevent future 'sieze-up' and 'suck back' of muffler water into the combustion chamber. (Get back to me on this and I will describe) You will need to modify the exhaust system so that the highest physical part of the exhaust system (between the exit of the exhaust manifold and the injection nozzle to the water lift muffler) ... is several FEET above the present elevation. This will build a large 'suction breaker loop' out of threaded black iron pipe and a new (aluminum) water injection elbow, and insulation 'lagging'. You will 'trash' the current 45 degree down-sloping Yanmar water injection elbow that is attached to your engine's exhaust manifold ... if your waterline is at or near the SAME elevation of the present Yanmar injection elbow ... you will have to build a 'dry stack' bewteen the engine and the water lift muffler to prevent future 'water suck back'.
The use of MMO to un-seize a frozen engine is the first attempt to remedy the problem. It doesnt require that a mechanic totally disassemble the engine and 'replace everything in sight'. 99 times out of 100 you can 'free-up' the engine with MMO. The caution here is that MMO is combustible so you have to remove as much as possible before you actually start the engine or you risk a 'runaway' (engine rapidly building to INFINITE rpm followed by loud crashing noises, engine parts coming through the side walls of the engine and maybe going though the hull and human bodies parts nearby .... keep that rag standing by when you attempt to start and the engine 'runsaway'. Keep that RAG nearby !!!!
