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Old 18-10-2020, 05:48   #1
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Yanmar 2GM20F Knocking sounds

We have a love/hate relationship to or 2gm20f engine and done quite a bit to keep it running like a clock.

Recently we've started to hear rather loud knocking sounds on certain RPMs, especially on idle (800) and up to 1200.

Since it's so rpm related I was thinking vibrations of some sort, something hitting something.
The shaft is straight, rotates with no problem by hand, gearbox and engine has recent oil. Propeller looks fine by the looks of it.

I thought I had found the problem when I found the alternator cable loose along the engine room wall but after fixing it to the wall with clamps still no luck.

The yanmar is in a Pearson 303, we've had her for four years and some 5000 miles. Only real problems have been related to fuel supply, contaminated tank and lines (replaced).

What's next to try? Would love to not just throw random parts at the problem. New engine mounts?
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Old 18-10-2020, 05:56   #2
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Re: Yanmar 2GM20F Knocking sounds

have you listened in the engine room where you might be able to know where it might be coming from?
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Old 18-10-2020, 07:05   #3
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Re: Yanmar 2GM20F Knocking sounds

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Originally Posted by gonesail View Post
have you listened in the engine room where you might be able to know where it might be coming from?
Yeah, I feel like it's towards the back, gearbox/back of engine or shaft, but I don't understand what that could be without seeing it hit something since the shaft rotates without noise when in neutral and I think it would've sounded different if it was from inside the gearbox?
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Old 18-10-2020, 07:09   #4
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Re: Yanmar 2GM20F Knocking sounds

Our Yanmar had similar symptoms. I finally had the local Yanmar dealer come out and look. It scared him to death. It was the motor mounts. The rubber was so soft that the metal pieces were actually hitting each other. The issue was mostly evident at low idle. If you raised rpm to about mid range it would mostly go away. Could be the aft mounts are worse than the forward. Also have noticed lots of other mystery rattles have gotten much less since the replacement. Suspect that the new mounts isolate the vibration that gets transmitted to the rest of the boat.
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Old 18-10-2020, 08:55   #5
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Re: Yanmar 2GM20F Knocking sounds

My 2GM20F on previous boat started knocking loudly as the cone clutch wore out... thought it was the folding prop at first (which a mechanic insisted it was, and we had balanced to no avail) but it steadily got worse, and would knock / shake until the clutch would engage. A burst of power would help until it got too bad for even that - once engaged it was OK until taken out of gear.



I actually drove the boat backwards through Canadian customs on my way to get it fixed. If it's better or doesn't happen in reverse (which gets less wear) that may be your problem. New cone and ancilliary bits is about a boat buck (or was, circa 1996).
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Old 18-10-2020, 09:01   #6
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Re: Yanmar 2GM20F Knocking sounds

ďKnockĒ happens while in gear only?
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Old 18-10-2020, 09:26   #7
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Re: Yanmar 2GM20F Knocking sounds

Quote:
Originally Posted by Geek_Guy View Post
Our Yanmar had similar symptoms. I finally had the local Yanmar dealer come out and look. It scared him to death. It was the motor mounts. The rubber was so soft that the metal pieces were actually hitting each other. The issue was mostly evident at low idle. If you raised rpm to about mid range it would mostly go away. Could be the aft mounts are worse than the forward. Also have noticed lots of other mystery rattles have gotten much less since the replacement. Suspect that the new mounts isolate the vibration that gets transmitted to the rest of the boat.
This is sort of my working theory too when I started googling about engine mounts and the rubber's lifespan. 4 years with us, twice up and down Bahamas to NYC with a good amount of motoring. Changing them seems to be an absolute pain in the ass, it's not like there's good access to the shaft/gearbox flange or the mounts. Any tips?

The knocking is after it goes into gear, and doesn't change after being under load, when you bring it down on RPM it'll come back even after hours at 2000 rpm.

How would you describe the clutch knocking Redline452? More like thumps, or like a big gauge electrical wire against wood (what we have)?
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Old 18-10-2020, 10:10   #8
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Re: Yanmar 2GM20F Knocking sounds

Possibly broken springs, loose bolts, or cracks in flex plate. Often sounds like a shot main bearing.
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Old 18-10-2020, 10:55   #9
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Re: Yanmar 2GM20F Knocking sounds

I might check my oil pump?
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Old 18-10-2020, 11:26   #10
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Re: Yanmar 2GM20F Knocking sounds

Had exactly the same problem - turned out to be the starboard rear mounting rubber had disintegrated intenally with no external evidence.
Fitted two 2nd hand mounts to the two rear ones.
Undid the top bolts on mounts, undo the gearbox mounting, heave the whole lot up, I used a low-level car jack on the gearbox drive-flange, after trying with a 4' crowbar.
Took 90' for the first and 10' for the 2nd.
Now replaced with a 3YM20 - 3 cylinders and 65% more power - a vastly better engine, driving an Autoprop.
A bit of a tight fit, but close enough to work.
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Old 18-10-2020, 15:09   #11
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Re: Yanmar 2GM20F Knocking sounds

I also vote for engine mounts as the main suspect.

The 2GM20 vibrates a bit at low rpm at the best of times and worse once the engine mounts are on the way out.

Be aware the mounts have different load ratings between front front and rear.
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Old 18-10-2020, 15:51   #12
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Re: Yanmar 2GM20F Knocking sounds

Quote:
Originally Posted by sveinung View Post
We have a love/hate relationship to or 2gm20f engine and done quite a bit to keep it running like a clock.

Recently we've started to hear rather loud knocking sounds on certain RPMs, especially on idle (800) and up to 1200.

Since it's so rpm related I was thinking vibrations of some sort, something hitting something.
The shaft is straight, rotates with no problem by hand, gearbox and engine has recent oil. Propeller looks fine by the looks of it.

I thought I had found the problem when I found the alternator cable loose along the engine room wall but after fixing it to the wall with clamps still no luck.

The yanmar is in a Pearson 303, we've had her for four years and some 5000 miles. Only real problems have been related to fuel supply, contaminated tank and lines (replaced).

What's next to try? Would love to not just throw random parts at the problem. New engine mounts?
Four years and 5000 miles, how many hours motoring in those 5000 miles, engine hours in gear?? Check your cutlass bearing. A diver can do it in the water, easy on the hard, a much less expensive fix than the others suggested. The cutlass bearing wears out, its rubber stabilizing the propeller shaft. When worn it allows vibration through the entire propeller shaft to the transmission/engine.
Best Iíd luck identifying the problem...
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Old 18-10-2020, 16:26   #13
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Re: Yanmar 2GM20F Knocking sounds

Quote:
Originally Posted by sveinung View Post
This is sort of my working theory too when I started googling about engine mounts and the rubber's lifespan. 4 years with us, twice up and down Bahamas to NYC with a good amount of motoring. Changing them seems to be an absolute pain in the ass, it's not like there's good access to the shaft/gearbox flange or the mounts. Any tips?........
All the more reason to suspect the mounts and/or shaft alignment.

If you can't do it due to poor access, it is highly likely previous owners did not do it either. They either hoped for the best or sold the problem on.

The shaft alignment changes as the engine mounts wear (become softer). The alignment needs to checked occasionally and the mounts replaced as they wear. Those mounts could be original and if so, they will be the principle suspect.
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Old 18-10-2020, 16:40   #14
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Re: Yanmar 2GM20F Knocking sounds

Quote:
Originally Posted by Telemark43 View Post
Four years and 5000 miles, how many hours motoring in those 5000 miles, engine hours in gear?? Check your cutlass bearing. A diver can do it in the water, easy on the hard, a much less expensive fix than the others suggested. The cutlass bearing wears out, its rubber stabilizing the propeller shaft. When worn it allows vibration through the entire propeller shaft to the transmission/engine.
Best Iíd luck identifying the problem...
On the hard, disconnect propeller shaft to transmission coupling, free up shaft seal either dripless or standard packing gland, pull the propeller shaft out.
Insert a suitable sized drift and slide hammer, pull out old cutlass bearing.
Insert new bearing, reinstall propeller shaft and transmission flange, realign propeller shaft to transmission...only when in the water as the hull shale is different when in the water. Typical yard cost about one boat unit.
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Old 18-10-2020, 16:44   #15
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Re: Yanmar 2GM20F Knocking sounds

Further to the access issue - as a generalisation and for older boats, the best place to look for potential problems is where the access is most difficult.

The reasons are obvious!
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