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Old 26-02-2018, 17:11   #1
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Yanmar 2GM20F knocking sound

Could someone help me diagnose this knocking sound coming from my 2GM20F engine? Halfway through the recording I am playing with the decompression levers to see if the sound changes.

1) When I lift the the decompression lever for cylinder 1, I get a pulsating feedback on my finger.

2) When I turn the crankshaft by hand, slowly, at a certain position, I can hear a spring sound, such as tsk-tsk-tsk, once per revolution.

Thank you,
SV Pizzazz
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Old 26-02-2018, 17:39   #2
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Re: Yanmar 2GM20F knocking sound

I have one of these. Love it, by the way. I am no mechanic, but I think that might be a lifter. It doesn't seem to be affecting the engine much, is why I think so.

My engine makes this same sound on start up. As the oil circulates, it stops. I always figured that was oil getting up to the lifters. Maybe you have oil gumming up a an oil passage?

You need a real mechanic, I'm afraid. I hope one chimes in here. Good luck.
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Old 26-02-2018, 18:06   #3
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Re: Yanmar 2GM20F knocking sound

Who knows if this might help but I just pulled the 3 injectors from my 3GM30F. Ine of the three was missing the o-ring that goes around the bottom of the injector. It seemed reasonable that this might have been the source of the clicking sound on my engine. The injector nozzles are all shot so they're still at the shop, so I can't test my theory, but I am super curious to find out soon...
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Old 26-02-2018, 18:16   #4
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Re: Yanmar 2GM20F knocking sound

Sorry, but I don't hear a knock. What I do hear is valve lash racket. That reminds me I still have to adjust my valve lash.
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Old 26-02-2018, 18:36   #5
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Re: Yanmar 2GM20F knocking sound

Lifter/valve adjustment. Starts and runs OK, besides the noise? If not, could be an injector or some inj pump internal mechanism headed south, but doesn't sound like it to me...
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Old 26-02-2018, 19:05   #6
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Re: Yanmar 2GM20F knocking sound

Well, I adjusted the valves twice. May be I am doing it wrong but I did everything according to the shop manual. Will try again. I can recognize the valve lash but it is what it is, may be one day I will overhaul the head.

The engine runs fine and especially when it warms up it has this nice, deep sound to it.

The sound that bugs me is this pa-pa-pa at low rpms that gets drowned at higher rpms. My old motor, a 2GMF was also doing it... May be it is just the way Yanmars are? I will check with me neighbour tomorrow to see if his engine does the same.

I appreciate your comments very much.
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Old 26-02-2018, 21:33   #7
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Re: Yanmar 2GM20F knocking sound

The good news is both cylinders sound about the same when you did each cylinder decompression. Two thoughts, First, perhaps an engine mount might be weak. Does the engine hop a bit with the transmission in gear.

Second, Could be a bit of valve lash. But I don't think there is much wrong with the engine. If the valves are all at 8 thousands and were adjusted cold (after sitting overnight), and the engine starts easy, it's most likely fine.
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Old 27-02-2018, 08:40   #8
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Re: Yanmar 2GM20F knocking sound

That sound is coming from the valve train. With the springs exerting pressure on the valves at different times it is hard to see. Remove the valve cover and first check to ensure that oil is indeed being pumped to the valve train. The valve stems and rocker arms should be wet with oil. Next, turn the crank to compresson for cylinder 1 and feel the rockers for looseness. They should move sideways on the lifter shaft. Then try rocking them up and down to check for vertical play. Check the gaps. It could be that one of the "buttons" that go on the valve tips has fallen off; that would cause a large gap and increased noise. (yes you will feel a pulsation from the decomp lever because it is interacting with the moving valvetrain inside!) If the gap (lash) is within spec, other things need to be checked. A valve could be sticking, causing the valve lash to expand. A compression test would determine whether that is occurring, but those are hard to diagnose otherwise because of pressure from the springs. The best way is to remove the head and disassemble the valve train, then check the valves for carbon buildup, both on the valve heads as well as on the shafts. Carbon buildup on the shaft can cause a valve to stick. Inspect the valve seats while in there for trueness; it is always a good idea to lap the valves. All the pieces from each cyilinder should be labeled and stored in their own cannister, to allow every part to return to where it was before teardown. This means each valve, spring, the little button, keepers for the spring and the disc that sits on top of the spring.
As the engine warms, the parts expand from heat, so the noise from the valvetrain diminishes as a result, but that valvetrain sound shouldn't be that loud at startup.
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Old 01-03-2018, 17:56   #9
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Re: Yanmar 2GM20F knocking sound

Sounds just like mine and has for years Go sailing and dont worry
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