Low RPM loud knock with Yanmar 3JH2E

[planoron]

First let me thank everybody for their help and information with everything to date. So to my next issue:

It's a 1999 Hunter 380 with a Yanmar 3JH2E 36 hp. Whenever the rpm's are around the idle speed (whether in gear or not), it has a very loud knock and seems to coincide with some vibration. The wonderful thing about the web is you can find too much info. Here is the scenario.

*Start the boat at idle: Loud knocking. You can hear it across the slips. Put the boat in gear at low rpm and you still have the loud knock. Bring it up to 1,000 rpm and it's gone. Does not matter whether it reverse or forward. I have yet to try raising it in neutral but have just assumed this is a harmonics problem. I will check this weekend.

Other Notes: no smoke, starts easily, fluids good, no oil use, flawless response with rpm's and shifting, 8" 2 blade prop, 1 1/4" stainless shaft, survey shows no signs of sloppiness or end play with cutlass bearing.

From Survey: Stuffing box and packing gland was bronze hex nut type. Boot was double clamped and serviceable. Monitor frequently. (Findings) Stuffing box is leaking more than 3 drops per minute after engine shutdown. (Recommendations) Investigate and repair as necessary.

Thanks in advance,

Ron

 

[rpwillia]

Check your motor mounts

My first thought would be you have a bad motor mount and at low rpm's the engine is a little off balance with the knock going directly to the hull.

 

[seadaddler]

Check

Check rockers and take value cover off and check for loose tappers,once had a chevy truck did same thing and had one or 2 worn tappers and replaced with new ones.
Look at the motor when idling and see any thing first that is worn like motor mount broken or loose belt or pump when low idle.
Nick

 

[Tim R.]

When were the valve clearances last adjusted? Did this start suddenly or gradually? Could be motor mount as stated above. I assume oil pressure is good and level is correct.

That Chevy truck probably had hydraulic lifters while the Yanmar has mechanical lifters. If it is a tappet, adjustment should resolve.

 

[Anonymous 121765]

Re: Low RPM loud knock with Yanmar 3JH2E - Hunter 380

Ron,

Both RP, Nick & I agree.

From your description, I would assume the "Very loud knock" you hear is either internal or engine mount. If it were your rockers, it would sound more as a tap. If it were a bent or broken lifter rod, this would give you the loud knock & vibration.

A worn out mount would have a "Thumping" sound. If the mount is worn & it's metal to metal, this could give a knocking sound.

You may want to pick up a mechanic's/engine Stethoscope at an auto parts store. They're pretty cheap. You will be able to check different areas of your engine while @ idle to locate the area where the knock is coming from. This will tell you alot if the knock is internal.

CR

 

[seadaddler]

Loud

Sorry if it is a loud knock than not tapers as said but try narrow down soon
as if a internal knock than finding it sooner is better before major damage
happens and sooner maybe can be repaired with rebuild.
If idle knock maybe than when more rpm's give more oil for better lubing
and bearing is worn.
My friend had a loud knocking but really bad loud banging on our way to Bahama's and needed to be towed and mechanic needed to drop the pan to see what was banging,it was a bad bearing and had it repaired.
maybe see if cutless bearing is bad and sound traveling up to motor,or if motoring shaking motor mount maybe bad,my motoring vibrates and shakes
at idle and than smooth with little more rpm in neutral.
Nick

 

[mr c]

Injector knock. Happened in my chevy diesel.

 

[Benny17441]

Re: Low RPM loud knock with Yanmar 3JH2E - Hunter 380

I second the thought that it sounds like a bad motor mount. I visual inspection will not tell you anything, most times they look brand new and could be cracked or too soft. Try rocking the engine by hand; you should not be able to move it much. Yanmars also like to idle kind of high at around 800 RPM and an undue low idle speed may cause a knock on partially worn but yet serviceable mounts.

 

[planoron]

Re: Low RPM loud knock with Yanmar 3JH2E - Hunter 380

Well, as with most things I do, they tend to grow. I will check out the mounts while a friend is topside on the helm. If nothing there it will be time for the mechanic to come out and I will add a couple more items to my list. The key and start button is down by your calf on the helm. That is fine but it also where my temp and oil alarm light is. Factor in that I worked 20 yrs in rock clubs so that little beeping noise is nonexistent even without any music going.

I have a ton of room on the helm so I am thinking I would actually like to see the temp and oil press instead of idiot lights where I'm not looking. Always been a gauge man.

 

[Anonymous 121765]

Re: Low RPM loud knock with Yanmar 3JH2E - Hunter 380

Plan,

Your engine manuf. should have updated panels and/or cables. It's only the panel size that will probably change & you may have to do some cable mods.

I had an old orig. minimal panel for my 5411 engine mounted down low at foot level. It had a buzzer & not much more. When I repowered, I bought the newer panel with gauges & mounted it higher up to my seat back perpendicular to the wheel. No one sits there anyway.

I would much rather see gauges than idiot lights. When an idiot light goes off, it may already have been too late.

CR

 

[kloudie1]

Re: Low RPM loud knock with Yanmar 3JH2E - Hunter 380

'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..

 

[jsalley]

Agree with Kloudie - had this happen on my previous boat after an engine mount bolt vibrated loose. When I tightened it, I started having the same symptoms (on a Yanmar 3YM); apparently, the bottom nut on the mount had also moved, and simply retightening moved the shaft too close to the inside of the shaft log.

 

[planoron]

'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..

Would this problem also be knocking in the neutral position?

 

[kloudie1]

Yes it can be. The engine jumps a lot at idle in neutral or in gear.. Can be an engine mount plate hitting or the shaft hitting the side of the tube.. With it idling, push hard on the valve cover to one side or the other to see if the noise changes significantly..if it does, it could be a shaft or mount hitting.

 

[charliegrau]

I've got a 1999 380. I replaced the motor mounts last year and it made a huge difference in the sound of the engine. If you haven't replaced yours, I recommend it. Before I replaced mine there was distinct "rattle" on start up, and noise was louder at idle than at speed. Now, it's like a car starting up...no drama, or noise...just "vroom".
-charlie

 

[badsanta]

knocking

After you check the mounts get a laser RPM checker $20 eBay. And check engine speed. It may be incorrect from your tach.it may be off several hundred RPM'S and you are actually be idling to slow.

 

[braddexter]

Hi
I had the same with a Yanmar 3GM.... it was pretty worrying, especially as it was only audible when I was coming in or leaving the berth.
I checked everything I could think of.... but it wasn't present when the reeves went up, so I upped the idle speed 100-150 rpm and it disappeared!

 

[nutheastah]

I would check my injectors first. a sticky injector can delay timing and cause an audible knock and can clear out under higher RPM. My guess though is something on top of the piston (most likely carbon). If the engine sat all winter and was fired up this spring carbon could have dropped into the combustion chamber from the back side of the exhaust valve and stick to the top of the piston. there is very little room between the piston and head usually less than .030 in. On a gasoline engine you would rid this problem by drizzling ATF down the carb while keeping the RPM's high. HOWEVER it is a risky thing to do on a diesel as the engine will burn ATF as if it were fuel and you would stand a good chance of a runaway. Also I believe that the newer Yanmars have pre-combustion chambers and how ATF would affect the engine I couldn't tell you. If the problem is an injector , having them tested will give you the answer. If the injectors test good it wouldn't do any harm to make up some kind of fitting or borrow a yanmar compression tester and while the injectors are removed pull your compression release while blowing compressed air into the cylinders. You might blow out the material through the open exhaust valve.
Bruce

 

[alain croteau]

I have same engine and when I first got the boat, it had what seems was similar loud knock. The engine jumped around so much that it had stripped the water lift muffler screw holes so muffler was banging against its base. Changed the mounts using Yanmar OEM and refastened muffler and knock is gone. Also installed a flexible hump hose. Engine still jumps around a bit 1000rpm. 1100 and up is good.

 

[misha93]

First let me thank everybody for their help and information with everything to date. So to my next issue:

It's a 1999 Hunter 380 with a Yanmar 3JH2E 36 hp. Whenever the rpm's are around the idle speed (whether in gear or not), it has a very loud knock and seems to coincide with some vibration. The wonderful thing about the web is you can find too much info. Here is the scenario.

*Start the boat at idle: Loud knocking. You can hear it across the slips. Put the boat in gear at low rpm and you still have the loud knock. Bring it up to 1,000 rpm and it's gone. Does not matter whether it reverse or forward. I have yet to try raising it in neutral but have just assumed this is a harmonics problem. I will check this weekend.

Other Notes: no smoke, starts easily, fluids good, no oil use, flawless response with rpm's and shifting, 8" 2 blade prop, 1 1/4" stainless shaft, survey shows no signs of sloppiness or end play with cutlass bearing.

From Survey: Stuffing box and packing gland was bronze hex nut type. Boot was double clamped and serviceable. Monitor frequently. (Findings) Stuffing box is leaking more than 3 drops per minute after engine shutdown. (Recommendations) Investigate and repair as necessary.

Thanks in advance,

Ron

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.