I recently purchased a JD 4620 which is generally in good condition. It has a total time showing around 5900 hours and seems to have taken good care of service. The engine has a loud knocking noise that starts at engine speeds above 2000 rpm. After the tractor has warmed up, it can be heard while the tractor is stationary at speeds above 1500 rpm. Listening to it with a stethoscope, it appears to be coming from the center of the motor in the upper half; Very little noise is picked up from the oil pan area. I thought maybe a valve adjuster had come loose, so I removed the valve cover and checked the clearances – everything seemed fine.
I’m looking for some guidance on what the fault pattern could be on this engine before ripping into it much deeper. I’m thinking maybe a fallen valve guide or a wrist pin?
Listen with the scope by touching each injector. Sometimes they cheat on you, so take them all to get checked. You’ll think it’s an injector only to find it’s two cylinders away from where the noise seems to be coming from. And whatever you do, don’t bend or force the lines going from the pump to the injectors.