Yanmar 3YM30 Knocking

[SV.StellaMaris]

Our Yanmar 3YM30 starts immediately and runs smoothly at idle till warm - about 5 mins in. At that point we are getting a distinct knock that seems to come every third vibration. If we raise the idle speed or run under power the knock smooths out. I’m not sure it disappears, but it’s much less noticeable, almost not noticeable at all.

Someone suggested raising the idle speed. But if there’s an actual problem, I’d prefer to address it rather than mask it. This is our first diesel and it usually runs terrific. I believe it’s time to change out our engine mounts based on their age. Although the engine runs smoothly otherwise and I don’t see any problems with them.

Could anyone familiar with these engines offer any feedback?

[Compass790]

Originally Posted by SV.StellaMaris

Our Yanmar 3YM30 starts immediately and runs smoothly at idle till warm - about 5 mins in. At that point we are getting a distinct knock that seems to come every third vibration. If we raise the idle speed or run under power the knock smooths out. I’m not sure it disappears, but it’s much less noticeable, almost not noticeable at all.

Someone suggested raising the idle speed. But if there’s an actual problem, I’d prefer to address it rather than mask it. This is our first diesel and it usually runs terrific. I believe it’s time to change out our engine mounts based on their age. Although the engine runs smoothly otherwise and I don’t see any problems with them.

Could anyone familiar with these engines offer any feedback?

Not familiar with that engine but I'd check oil pressure, screw mechanical gauge into where sender is if the motor doesnt come with an oil pressure gauge. How many hours on the engine?
I'd agree with finding out why it's started to knock, dont ignore it.
Over to 3ym30 owners now.....

[Lepke]

A bad injector can make a knocking sound. Depending on how much fuel is getting thru the injector the knock can be loud enough to be confused with a rod bearing knock. I don't know your injectors, but they usually have a needle valve and a spring. Incoming fuel pressure lifts the needle valve to allow fuel to spray into the cylinder. Spring pressure closes the valve to stop combustion debris from getting into the injector.

Put some towels around the injectors to catch diesel. With the engine running, loosen the top injector tube nut, one at a time and listen for a change. No change = bad injector. Often the injector can be removed, disassembled and cleaned for improvement. Or take them all to an injector shop hand have them rebuilt.

[skipperpete]

There are many things that can cause the knock you describe. Doubtful that it’s a conrod bearing and on your engine I suggest you try ..... in neutral.... run the engine up to 2,000 rpm then back off quickly to idle, a bearing rattle will be louder on backdown due to no fuel on power stroke but a diesel knock will disappear.
Consider the possibility that the knock is related to the engine mounts ie a broken adjusting bolt or a shaft rattle where the alignment has put the shaft so close to the stern tube that at some points of the rpm range it taps on the inside of the sterntube , to check to see if this is the case physically push the engine a bit with your foot and see if the noise abates.