Mercruiser 5.0 MPI hydro locked

[salty3rd]

All, during a 1/2 hour ride on Sunday I noticed a little sluggishness in this engine. It then seemed to clear up, and when I came down to an idle the RPMs were erratic so I shut the engine down. I checked a couple things and when I went to restart it was hydro locked. I pulled all the plugs and water came out of some of the cylinders on both banks. Oil is also milky. The engine is raw rather cooled in salt water, manifolds and risers are 3 years old. The flappers were new this Spring. I’m guessing a bad intake manifold. I removed the intake manifold but haven’t found a smoking gun. Any thoughts on how to move forward are appreciated.

 

[nola mike]

All, during a 1/2 hour ride on Sunday I noticed a little sluggishness in this engine. It then seemed to clear up, and when I came down to an idle the RPMs were erratic so I shut the engine down. I checked a couple things and when I went to restart it was hydro locked. I pulled all the plugs and water came out of some of the cylinders on both banks. Oil is also milky. The engine is raw rather cooled in salt water, manifolds and risers are 3 years old. The flappers were new this Spring. I’m guessing a bad intake manifold. I removed the intake manifold but haven’t found a smoking gun. Any thoughts on how to move forward are appreciated.

Pressure test the cooling system, either replacing the manifold or blocking off the intake ports.

 

[Scott Danforth]

acetone test the exhaust manifolds

 

[Lou C]

If the intake water crossover passage rotted thru or if the gasket leaked that would put water in the cam valley but not the cyls; so water would show up in the motor oil first. If you have water in the cyls the exhaust is a much more likely cause. If you remove the elbows after draining the exhaust it will be obvious if the exhaust has leaked back in the cyls.
The other thing that can cause it is an overheat, this can damage cyl heads (cracks) or cause a head gasket to leak. Which cyls had water in them? If it was 3 & 5 and 4 & 6 could be from that manifold to elbow joint. Any rust stains there? A 2000 model year is still a wet joint exhaust style not the newer dry joint….

 

[salty3rd]

Update; I pulled the starboard exhaust manifold and riser and found no evidence of leakage. Inspected the heads and found evidence of saltwater in cylinders 1,3,5,7 and 2.in the intake ports. (5&7 were slightly more evident). I’m guessing my next step it to pressure test coolant system as Mike suggested. Should I just pull the head and have it pressure tested at a machine shop at this point since it’s already mostly apart? Any thoughts are appreciated on the best way to move forward..

 

[tpenfield]

Could be intake manifold or cylinder heads, if the exhaust side checks out OK. I know the 7.4's had some issues with rust-through around the valve guides, not sure about the 5.0 engines.

Your profile says that you are from Plymouth, MA . . . I'm assuming it is salt water boating. If the engine is 20+ years old, it may be done. I got 20 years out of a 7.4 carb'ed engine in the salt . . . then it was 'done'.

 

[cyclops222]

Save yourself a lot of ...What iffs.... and money. Save the season ?
Buy a NEW motor. Not a rebuilt pile of CLAIMED new parts.
Buy NEW only. Good luck.