Mercruiser 3.0L Knocking Sound

[Kevin F]

Folks,

Just bought a Maxum Bowrider 1990 vintage with what appears to be the original engine (5000 Hrs) and on my first outing engine ran very smoothly but after about 20 minutes power dropped and a loud knocking sound came from the engine although it was still running. Engine starts fine oil pressure is good but when revving in neutral noise comes from the back end almost as if the fly wheel was loose or something broken ?

Anyone have any suggestions or have a similar experience ?

[mmwjr]

Welcome aboard

5000 hours on a gas engine! Do you mean 500?

Coming from the back of the engine I would say gimbal bearing or joint however neither would cause a loss of power.

What oil pressure does it have? I'm thinking you may have lost a cylinder. Perform a compression test. If oil pressure is good then probably a rod bearing.