Low RPM loud knock with Yanmar 3JH2E

[misha93]

Ron,
No one in this discussion has mentioned about a vibration. If vibration occurs that is definitely means faulty engine mounts. Try to figure it out. In mine 2QM20 vibration was along with nocks and it was so great that some biminy fastenings gone loose. After replacing one mount everything got back inline.

 

[RichH]

When was the last time you checked the injector pump timing? Normally done every 500 engine hours on most Yanmars.
Then condition check / rebuild of - injectors, then finally the injection pump.

 

[Lorin Smith]

Tappet noise does not go away with increased engine speed. I cannot think of an internal engine problem that does, so move on to something else. Did this start suddenly? If yes, what have you recently done near the engine? Can a control cable be merely slapping the underside of your cockpit, or anything along this line? Yanmar recommends bi-annual replacement of the engine mounts (with no hour spec given). Since most of our mounts are "just coming up on two years old" look into POLY-FLEX mounts, and don't be squeamish about spending a few extra dollars on them, you will be happy to have them even in the event that mounts are not the source of the noise. Good luck!

 

[planoron]

Judging from the posts it sounds like I have a couple issues. Mounts and idle adjustment to start with. I found a graph of the engine vibration on a yanmar mounts site. When I drop the rpms down you can certainly feel the vibration of engine along with the knocking. Also, the knocking is is not a quick, light, tapping noise. Its loud.

Example: Coming back in, turn into the wind and fire it up in neutral. Starts perfect, whole lotta shakin goin on and knock and vibration dies as soon as I move into gear and out of idle. Coming to the slip and I back off the trottle to idle and the vibrations and knock appear. Round up on the slip and throw in reverse with rpms: smooth and quiet.
Back off to idle and we are shakin' and knockin'. I'm trying to get in touch with PO on general service questions but from what everybody says it certainly sounds like the mounts and idle adjustment are the place to start.

 

[Tim R.]

Check the belts. I did a top end rebuild on my 4jh2e and I reinstalled the belt because it only had about 50 hours on it. I started he engine after the rebuild and it made a horrible loud knocking noise that had me quite worried. It went away at higher rims . Turns out the belt had a slight cut in it and was hitting something on the front of the engine. It made the exact same noise as a thrown rod beading.

 

[robertsapp]

Late Injector

There's a good chance you have a late injector. Try this simple test:

With the engine warmed up and idling, open each compression release in turn. The knock will most likely stop when you open the release on the affected cylinder. If so, that means you're not dealing with any kind of engine mount/shaft/alignment issue, it's an engine issue.

If you can make the knock go away by opening the compression release on the affected cylinder, then try the next step. With all cylinders operating, engine warmed up and idling, rapidly give the engine full throttle. If the knock gets worse before it gets better, it's probably a worn bearing.

Once the engine is running at full RPM, rapidly pull the throttle back to idle. If the knock gets worse, it's probably piston slap (piston is loose in the cylinder and clatters around).

If though, as I suspect, your knock at idle goes away when you're at higher RPM, and neither of the two actions above cause it to get worse, you probably just have a late injector. The best way to be sure is to swap injectors between cylinders and see if the problem follows the injector. But the easiest thing to do is to pull the bowl on your primary filter, fill it with injector cleaner, reinstall it and bleed the filter, start the engine, and run it for about a minute or so to get the cleaner into the injectors. Then shut the engine down for about a half hour. I bet when you restart it your knock will be gone.

Good luck.

 

[Maine Sail]

'nother thing to check is the centering of the prop shaft in the packing box mount. The Yanmar 3 cylinder engines jump a good bit at low rpm .. if the engine was aligned so that the prop shaft is close to the "stern tube", it can bang quite a bit at low rpm when the bouncing engine causes the shaft to strike the inside of the tube. Weakened mounts can make this worse..Before doing anything else, check the shaft-to-transmission alignment to be sure the shaft is centered in the tube..

Ditto.. I see this quite often....

 

[Steve Dion]

I would also check the idle speed. Your tach may be off, so checking out the actual RPM is a good idea.

 

[Dubo]

I read alot of good ideas. However, what you want to do do is isolate the source. There are two variables at work here. Engine/trans area and what is outside the hull. Your first action should be to prove your engine ok. How? Where your engine couples with the shaft, decouple it. (four bolts probably) Separate the coupling. Start engine while keeping the shaft coupling apart. Is sound gone? Yes, engine is fine. Turn off engine. Reattach coupling.
Now grab the shaft where the coupling is and move it up, down, port and starboard. If one direction gives you a tap easier than over directions, your shaft is not centered in the shaft log. If you dive, you can probably visually inspect the shaft and see it is not centered. Bring an ice cream stick with you and a feeler gauge. As engine mounts age they will obviously alter the engines angle. My guess is that is what you are experiencing. Note that i did said "guess". Cutlass bearing I believe you stated is ok.
Happy hunting!

 

[Randy Church]

Check to see if transmission is shifted into gear 100%. Asked similar question years ago regarding the knocking I was experiencing at low RPM that would then disappear as I powered up. One reply said that 99.9% shifted into gear wasn't good enough. Adjusted transmission cable and sure enough, that was it and it's been years with no knocking. Good luck,
Randy

 

[redhead78]

I am with Robert and Maine on this also, a late or dirty or bad injector can sound like a rod bearing, i think the key here is it lessens with RPM increase, where a rod bearing or journal are going to get louder or bigger wf rpm increase. Red.....

 

[SeaMoore]

Please start with the simplest; what do you consider "idle speed"? I had the same problem with my Yanmar and mechanic I was I struggled to find the answer thinking all small engines idle around 750 RPM. Once I looked in the manual and set my idle at 900 RPM I never had the knocking and shaking again.

 

[Pacific Mariner]

If you have a knock, I think the advice on here, and the internet, is more expensive than having a qualified mechanic look at it. You just might save yourself the price of a motor...

 

[DStegs49]

I had a bad knock up through the rpm range. It was so bad that you couldn't hear inside the cabin. The hull and deck acted like an inside of a drum and amplified the sound. First, I replaced the cutless bearing because of play in the bearing. The noise was cut back to the mid range rpm's. After looking at it while it was running and pulling and pushing on different parts of the engine and drive train, I was able to deduce that the stuffing box/ bearing was the culprit. Or more precise, the motor mounts which caused the shaft to wear and vibrate against the lower part of the cutless bearing. I aligned the prop shaft by adjusting the motor mounts so the shaft went through the center of the bearing and the problem is now gone. Good luck with your problem.

 

[planoron]

To everybody: Thanks once again for all the input and support. I got with a couple guys and here is the verdict. Motor mounts and shaft alignment. We determined I am getting a little rough on the idle but the engine is fine and we r going to chalk most of that to old fuel. Tomorrow I will be doing a lot of motoring about just to burn off the old stuff. When we applied just a bit of pressure to the shaft it went quiet as could be.
It will be good to get that off the list because it looks like the 12v freezer just became the next item