I have a 1997 mercruiser 3.0lx that knocks pretty badly at around 2000-3000 rpms. It is almost undetectable at idle and at rpms above 3000. My initial thought was a rod bearing. There is good compression in all cylinders 155 for cyls 1&3 and 165 for 2&4. oil is clean. I had a mechanic try and screw me out of a new head on the initial diagnosis of why the engine lost power underway. he had taken the rocker arms and pushrods off when i pulled the plug on having him repair it. He didnt check the timing or spark or anything else first before he made the damaged head call. i asked around to several mercruiser shops and they described the protocal of troubleshooting so i realized he was trying to "get" me. anyways, after We put the head back together we bench tested all ignition & fuel flow with no problems. Timing was fine after checking to see if the timing gear may be broken (i was told that they used to put a nylon timing gear set in this engine but we found a metal one installed on this engine and all was correctly lined up with the two dots touching when lined up). We didnt have the compression in any cylinder and realized we tightened the rockers too much. after that was remedied, the engine started right up. thats when we heard the knocking as described. it is most certainly an interior engine sound that follows the rpm of the engine. I have no idea how many actual hours are on the engine. We purchased the boat used in 2003 and it is a 1997. We have put around 250 hours since then mostly run about hours but some tubing. I would be shocked if this boat has more than 600 hrs on it but cant be sure. Is that a lot of hours? And what could cause the rod bearing to fail if its not alot of hours? I know from adjusting the valves on the head that the oils is getting to the top of the motor (it is really messy to adjust the valve lash) so oil flow and pressure is very good and the gauges represent that.
Also are there any other things that can cause a noise like this?
I need some advice on pulling the engine and doing a lower engine rebuild. should i replace the oil pump/water pump/fuel pump/ front and rear seals/ have the crank refinished and the rings done? or is the compression good enough to just replace the rod bearings and seals and not the rings?
Any info is much appreciated.