4.3 Mercruiser V6 Bad knock from front

[Chain_skipper]

Hello,

Engine is a Mercruiser 4.3L V6, model year 1990 I believe. (S/N 0C665406)
Alpha One drive (S/N 0C684712)

Background information - The last time that I had her out the previous summer I was running near WOT. I trimmed the drive up a little too far, and the engine speed went about 3-400 RPM above the top of the range. Shortly after the engine had a knocking sound coming from the front, so I took it back to shore and then home.

That was the last run of the year, so I began to winterize it. I tried to troubleshoot the noise before I put away, so I changed the oil and filter, and started it on the muffs. The noise was not very audible at idle, but distinct around 2-2500 RPM, coming from around the harmonic balancer and water pump area. Took off accessory belts and ran briefly, the noise was still there. I then removed the pulley and ran briefly, and the noise went away. I then put the boat in storage for the winter.

After doing a bit of research, I found that a bad harmonic balancer can sound a lot like a rod knock. So I purchased a new one this summer and installed it. I started it this week and the noise is still present, and now sounds worse. I can hear the knock at idle up to around 2500, I don't want to go higher on the hose.

Can anybody suggest troubleshooting procedures or advice? I have so far not been able to come to a definitive conclusion, and I don't want to just throw money at it.

Thanks in advance

 

[alldodge]

If its a actual knock and not a tick then its rebuild time. A knock gets louder as the rpm increases, and if it does then probably spun a rod bearing

 

[Chain_skipper]

As much as I wanted it be a simple repair, the #1 connecting rod bearing was destroyed. It wasn't spun, but mashed onto the rod. Machine work is done, re-assembly will commence this weekend.

 

[Bondo]

the #1 connecting rod bearing was destroyed. It wasn't spun, but mashed onto the rod.

Ayuh,.... That sucks,...

Hard to believe it came apart at only 5200 rpms,....

 

[alldodge]

Probably cannot verify what all happened, and its mute at this point, but I have a Merc reman 454 MPI in the barn with a spun bearing. One of these days it might get rebuilt. If the crankshaft journal is not perfectly square, with the crankshaft, it will spin the bearing, and can be quick or take a few hundred hours. The one in the barn had only about 120 hours on it. So as the knock started it was slowed down and shut off. If it was run a bit longer under heavy load it would have smash the bearing.