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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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Old 10-13-2011
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Yanmar 3GMD vibrations - what is normal?

I have a 3GMD with new shaft, cutlas, engine mounts, fully "tuned" timing/valves, etc. At idle, and up to around 1800 rpm, it is really quite smooth for a diesel. However, from 1800 through to about 2350 rpm, it has a very pronounced vibration both under load and in neutral. From 2400 through to max 3600 under load, it is very smooth again. My question is whether there is a built-in vibration period that is not going to be eliminated by any tinkering, or does this sound like something that can be improved if I find the right culprits? The engine has perhaps 500 hours total, starts and runs great, burns no oil and minimal fuel, no discernable smoke of any color. The vibration existed before the shaft/mount work which was done last winter.

Thansk for your thoughts,
Mike
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Old 10-13-2011
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Check the compression and then the fuel injection to all three cylinders. I suspect a dirty fuel injector.
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Old 10-13-2011
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This sounds *EXACTLY* like the issue I had with my Yanmar 3ym30 when I bought my boat new. The same smooth / rough / smooth issue!

The diagnosis for my boat was that it was 1 single bad motor mount. Dealer replaced it and its been smooth since.
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Old 10-16-2011
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Thanks for the inputs. It sounds as if you wouldn't expect a vibration period in the middle of the RPM range, so I know i need to keep checking. All ideas welcomed.

Mike
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Old 10-17-2011
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Quote:
Originally Posted by mbsl98 View Post
Thanks for the inputs. It sounds as if you wouldn't expect a vibration period in the middle of the RPM range, so I know i need to keep checking. All ideas welcomed.

Mike
I have samd 3GMD (1980 vintage). It runs smooth from idle to max rpm of 3,300 on the yanmar tach. Not sure why I cannot get to 3,700 rpm (I did manualy push throtle control at engine to max otherwise I only get to 2700 rpm). If it happens both in gear and in neutral, must be somthing to do with the engine I would think. If it were filters, you would think after a certain rpm you would have vibration problem, not in the mid range (worth a try to replace all filters however). As far as fuel, it can only be filters, lift pump, injection pump, or injectors. If the engine has not been run much, I would make sure you have clean new fuel and maybe just put some hours on her (under load) and maybe she will clean herself up.

I just checked the 3gmd manual, 3300 rpm is the normal long term rpm recommended. Therefore I think I am ok.

The motor mounts comment above makes a lot of sense- when you hit a certain harmonic it could start to vibrate at a certain rpm with a bad motor mount. You should be able to see the vibration- have someone operate throttle while you look at the engine shaking.

BTW, I get a grey smoke at high rpm in gear, all seems to run well so I do not know if this is a problem or not, or maybe just steam from the raw water mixing at the elbow. Is your exhaust completely clear?
Regards

Last edited by casey1999; 10-19-2011 at 03:45 PM.
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Old 10-18-2011
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Thanks, Casey.
My boat was just hauled for the season last weekend, so this will be an ongoing search next spring, but now I know there is something to find, that it's not just the way the 3GM runs. Mounts are all new, and it was similar with the old mounts, so that is probably not the problem.

My exhause is very near to clear. Sometimes there will be some white (very light grey?) vapor, but that is air temp and humidity related, not internal to engine. You might be seeing the same when you say a grey exhaust. If it disipates quickly in the air,it is probably moisture, nothing to worry about if not excessive. Excess fuel burns quite dark/black, and oil burning tends to be blue smoke.

For your engine RPM's, if you can't reach 3300 by pushing the throttle, but can manually, it sounds like there is a problem with the linkage not letting the control at the engine go all the way to the end of its travel. Separately, not being able to get to 3600 (rated max) is very likely a problem of having a slightly too big or too pitched prop, or bottom fouling if it used to reach 3600 and doesn't now. Prop is usual culprit when not hitting max consistently. Do you know if it used to get to 3600 and has dropped over time? It is not a big deal to get the prop pitch altered a small amount, and you are not way off now. It will run better, longer, if it can reach its full range under full power.

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Old 10-18-2011
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Quote:
Originally Posted by mbsl98 View Post
Thanks, Casey.

My exhause is very near to clear. Sometimes there will be some white (very light grey?) vapor, but that is air temp and humidity related, not internal to engine. You might be seeing the same when you say a grey exhaust. If it disipates quickly in the air,it is probably moisture, nothing to worry about if not excessive. Excess fuel burns quite dark/black, and oil burning tends to be blue smoke.

For your engine RPM's, if you can't reach 3300 by pushing the throttle, but can manually, it sounds like there is a problem with the linkage not letting the control at the engine go all the way to the end of its travel. Separately, not being able to get to 3600 (rated max) is very likely a problem of having a slightly too big or too pitched prop, or bottom fouling if it used to reach 3600 and doesn't now. Prop is usual culprit when not hitting max consistently. Do you know if it used to get to 3600 and has dropped over time? It is not a big deal to get the prop pitch altered a small amount, and you are not way off now. It will run better, longer, if it can reach its full range under full power.

Mike
My engine does not burn any oil, and exhaust color (white/grey) disipates within 3 feet of outlet, I leave it at that unless the engine shows some other problem.

The throttle linkage will not let it go to the max position at the engine- I will need to see if I can adjust.

I bought the boat used, and the engine has been in the boat for probably 20 years or more. The boat has had several owners and no one really could give me a good history. Even in neutral the max rpm is 3300. Note this is on the Yanmar tach. I have a photo tach, but had not yet had time to verify. I've heard Yanmar tach can be 300 rpm less than actual so seems I might be in the ball park.

Interesting thing is that in forward gear, I hit hull speed at about 2700 rpm (with clean hull and clean prop). So I guess this means prop is over pitched right? I should pitch prop to hit hull speed at say 3700 rpm.

One thing I did this past weekend is hooked up a fitting to the inlet coolant water where I can attach a bucket with a hose. I fill the bucket with fresh water and flush the engine at the dock. I am hoping by having fresh instead of salt water in the engine while sitting unused the engine will last longer.
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One other thing I did on my Yanmar 3GMD was to replace the anodes. Not sure if you have done this, but the plugs can be hard to get out (there is another thread here that covers my work). There is one anode in the head and two in the block.
http://www.sailnet.com/forums/diesel...gs-engine.html

Last edited by casey1999; 10-18-2011 at 06:39 PM.
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Old 10-20-2011
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Thanks, casey. I did replace the anodes last year, almost. I missed that there was a third behind the fuel filter, sothat is on this winter's list.

On yours, the ratings I have in the manual are 3400 max continuous, and 3600 max for not over 1 hour. 3700 does not appear. The Yanmar tachs are known for being off - don't know if that is always to the low end or not, but I think most references I have seen are for ones that were low. I found that my tach case had completely rusted through with 1/8" + holes through in a lot of places, but it still read pretty well. The new one reads a little higher, but not by over 100 rpm or so. I also changed out the three engine wiring harnesses, from the back of panel to all of the engine connections. The back of the panel, where the tach rusted, had a cou0le of bad connectors in the plugs as well, and I found a couple of bad connections inside plugs in the engine bay that looked perfectly good until i pulled them apart. Overall, it turned out to be good preventive maintenance, and all of the parts were in stock at Torrenson. My panel is out in the open in cockpit, and the seal wasn't good after 25 years.

I'm not sure I agree with pitching the prop to hit hull speed at 3600. In my case, I can push boat beyond hull speed by close to a full knot (7.5 with a 21' waterline). Usual advice is to pitch it so that you reach max rpm target of 3600 at max throttle. That would let you reduce rpm's to say the 3400 max for continuous running and still be around full hull speed, for instance. My boat is a little unusual in that it has so much power for a small boat, but the basic physics rules still apply. However, pushing beyond hull speed is really inefficient, as consumption jumps, as does teh size of teh wake, teh depth of teh standing bow wave, etc. In your case,m I'd check teh tach before relying on it for max readings.

There could be other causes for teh engine not being able to reach over 3300. For instance, blockage building in teh exhaust elbow where teh water enters the exhaust. That is a classic ppoint where the inside space gets reduced by carbon adn teh resulting back pressure reduces the max rpm.

Mike
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Quote:
Originally Posted by mbsl98 View Post
Thanks, casey. I did replace the anodes last year, almost. I missed that there was a third behind the fuel filter, sothat is on this winter's list.

On yours, the ratings I have in the manual are 3400 max continuous, and 3600 max for not over 1 hour. 3700 does not appear. The Yanmar tachs are known for being off - don't know if that is always to the low end or not, but I think most references I have seen are for ones that were low. I found that my tach case had completely rusted through with 1/8" + holes through in a lot of places, but it still read pretty well. The new one reads a little higher, but not by over 100 rpm or so. I also changed out the three engine wiring harnesses, from the back of panel to all of the engine connections. The back of the panel, where the tach rusted, had a cou0le of bad connectors in the plugs as well, and I found a couple of bad connections inside plugs in the engine bay that looked perfectly good until i pulled them apart. Overall, it turned out to be good preventive maintenance, and all of the parts were in stock at Torrenson. My panel is out in the open in cockpit, and the seal wasn't good after 25 years.

I'm not sure I agree with pitching the prop to hit hull speed at 3600. In my case, I can push boat beyond hull speed by close to a full knot (7.5 with a 21' waterline). Usual advice is to pitch it so that you reach max rpm target of 3600 at max throttle. That would let you reduce rpm's to say the 3400 max for continuous running and still be around full hull speed, for instance. My boat is a little unusual in that it has so much power for a small boat, but the basic physics rules still apply. However, pushing beyond hull speed is really inefficient, as consumption jumps, as does teh size of teh wake, teh depth of teh standing bow wave, etc. In your case,m I'd check teh tach before relying on it for max readings.

There could be other causes for teh engine not being able to reach over 3300. For instance, blockage building in teh exhaust elbow where teh water enters the exhaust. That is a classic ppoint where the inside space gets reduced by carbon adn teh resulting back pressure reduces the max rpm.

Mike
This past weekend I replaced some old exhaust hose and also inspected the exhaust/mixer elbow- it all looked clean with no blockage. When I get a chance I will measure rpm with photo tach. I did need to re make some of the engine wiring connections due to corrosion, my panel is inside boat so somewhat protected from the elements.
Good luck with your 3gmd
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