1992 Mercury 75HP outboard 2-stroke --> zero compression on bottom cylinder???

[sprinter]

The engine runs, but poorly. The other 2 cylinders show 115psi compression. There is a distinct knock in the motor. My thoughts are that it is probably the rod bearing. What do you guys think? Do I take the head or the crankcase cover off first to investigate? If the rod bearing is bad, would I not still get some compression reading?

 

[racerone]

There is no cylinder head on that motor.----Remove the exhaust cover on the side ( 20 bolts ) and have a look in there.

 

[sprinter]

There is no cylinder head on that motor.----Remove the exhaust cover on the side ( 20 bolts ) and have a look in there.

Thanks for the tip racerone, but will I be able to see the integrity of the rod bearing through the exhaust port? Also, If going through the intake/crankcase side of the motor, would I be able to change rod bearing without removing the entire powerhead?

 

[racerone]

Take the cover off to have a look at pistons through the exhaust ports.-----Rotate the flywheel when you have the cover off to check bearing play.-----That should tell the story.-----------------If you think you can rebuild this motor / correct this problem without taking the powerhead off you are mistaken.------If you can do it you are guaranteed a job at any outboard shop !!

 

[sprinter]

Take the cover off to have a look at pistons through the exhaust ports.-----Rotate the flywheel when you have the cover off to check bearing play.-----That should tell the story.-----------------If you think you can rebuild this motor / correct this problem without taking the powerhead off you are mistaken.------If you can do it you are guaranteed a job at any outboard shop !!

Thanks again racerone! I will follow your advice and report back. My intuition tells me that I will be doing a full rebuild... But I seriously suspect a bad rod bearing though. If it proves to be the case, would it not be possible to change the rod bearing from the crankcase side without pulling the powerhead? If not, why? My rationale is that the motor is not supposed to have many hours on it. That said, if one bearing is bad, it doesn't mean the others are. Plus the solid compression on the other cylinders suggests a fair condition. I would hate to do a full rebuild if other components are sound. Also, another thing that bothers me, if it is just the bearing would I not be still getting some compression? How do I explain a zero reading? I appreciate any insight.

 

[racerone]

The 0 compression is not a problem with a rod bearing.--------It is a problem with the piston and piston rings.--------------You are moving / thinking way ahead of where you should be !----Remove that plate and it will tell the story.---A bad rod bearing would also mean a bad connecting rod and a bad crankshaft.---Trust me it is not a simple fix !!

 

[sprinter]

Right you are racerone! After a full and lengthy teardown the culprit is a scorched piston and seized piston rings. Not exactly sure what would have caused this situation but clearly that's the issue. The rings were crushed on one side of the piston where the damage was located. The rings were squeezed (crushed) in the piston groove thereby unable to create a seal, hence the blowby and zero compression. Here is a link to a photo: http://i1383.photobucket.com/albums/ah306/sprinter2/100_5987_zps00a23f39.jpg.

The blackened area on the piston skirt below the rings is actually a tear in the piston. Approximatelly 1/32" of material is missing from the piston. What are your thoughts?

 

[sprinter]

The 0 compression is not a problem with a rod bearing.--------It is a problem with the piston and piston rings.--------------You are moving / thinking way ahead of where you should be !----Remove that plate and it will tell the story.---A bad rod bearing would also mean a bad connecting rod and a bad crankshaft.---Trust me it is not a simple fix !!

Right you are racerone! After a full and lengthy teardown the culprit is a scorched piston and seized piston rings. Not exactly sure what would have caused this situation but clearly that's the issue. The rings were crushed on one side of the piston where the damage was located. The rings were squeezed (crushed) in the piston groove thereby unable to create a seal, hence the blowby and zero compression. Here is a link to a photo: http://i1383.photobucket.com/albums/...ps00a23f39.jpg.

The blackened area on the piston skirt below the rings is actually a tear in the piston. Approximatelly 1/32" of material is missing from the piston. What are your thoughts?