Engine vibration universal m3-20

Jun 15, 2023
13
Catalina 28 Santa Barbara
I got an universal diesel engine M3-20 3 cylinder diesel running rough on neutral when idle around 1100 to 1200 rpm. Anything above 1300 to 1400 rpm runs perfectly. Since I got it this what has been done:
Replace all engine mounts with new cushifloat mounts.
Replace Racor filter and fuel filter and fuel pump filter.
Remove all diesel fuel and put new star tron tank cleaner and empty 2 times tank.
Pour new diesel in tank.
Flush fuel system with liquid molly diesel purge, notice a little bit less of vibration when running with liquid molly but nothing significantly.
Will a bad injector be causing this problem of rough idle and work OK? around 1300 to 1500 rpm? or this could be something else I should be looking into?
Also I read this could be the nature of the engine being a 3 cylinder. I will try to do a couple videos to shown what I am looking at.

Thanks in advance.
 
Jan 4, 2006
6,735
Hunter 310 West Vancouver, B.C.
Same thing I have at 1200 RPM with a 1999 Yanmar 2GM20F. The engine is vibrating at a specific frequency called a harmonic. Most engines have this. Just stay away from that RPM as best you can and you won't have a vibration problem.
 
Jan 11, 2014
11,883
Sabre 362 113 Fair Haven, NY
In general engines with fewer cylinders run rougher at idle than those with more cylinders. A 1 cylinder 4-stroke with fire every other rotation, a 2 cylinder engine will have a cylinder fire on every rotation, a 4 cylinder will have a cylinder fire every half rotation. At high RPMs the it doesn't make much difference because the cylinders are firing so quickly, at low RPMs the time lag between cylinder firings is longer so the vibration is more pronounced.
 
Jul 8, 2012
134
Catalina 36 MKII North East
It is common for the M3-20 to run rough at low rpm. Idle is about 800 and from there to 1000 it rattles. Your tach may be reading incorrectly and the engine is behaving normally. You can check the tach against a cheap handheld digital tachometer.
Another possibility is a clogged injector, they are all supposed to spray a similar pattern. Injectors operate at very high pressure. Rather than mess with them you can send them out to be serviced by a shop for a reasonable fee. Replacing injectors can get expensive.
 
Jul 5, 2011
727
Oday 28 Madison, CT
Brazen is right. I bought a 10 year old O'Day 28 in 1995 (Universal M12) and it would not idle without raising the RPM. Apparently the original owner had lost his water pump, overheated it and "coked" the injectors. I pulled them and sent them to Yankee Diesel in Danbury, CT and never a idling problem thereafter. Did not cost a bundle and so if you have an engine with significant hours an injector rebuild probably makes sense anyway, even if you do not get 12 cylinder Mercedes smoothness out of the deal.
 
Nov 6, 2006
9,952
Hunter 34 Mandeville Louisiana
Also, check the valve clearance. Probably has never been done since people get used to the hydraulic lifters in automobiles and don't think about mechanical valves. Not saying that is the problem, just suggesting a check.. 'cause that may be a problem.
 
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Jun 15, 2023
13
Catalina 28 Santa Barbara
Thanks guys for great advise. I did install diese tiny tach because was not sure if tachometer was reading correctly and got same reading I am about 1200 rpm both tachs. I will work on injectors and go from there. I know there was a problem with pump because it was replaced before I got it.
I think 1 of the injectors is not working 100 percent.
 
Jan 4, 2006
6,735
Hunter 310 West Vancouver, B.C.
Will a bad injector be causing this problem of rough idle and work OK? around 1300 to 1500 rpm?
No. An injector does not perform better when it runs faster.

I think 1 of the injectors is not working 100 percent.
I don't understand. Everyone has told you that all small diesel engines have a rough spot at low RPM. Mine had the same performance from brand new and no difference 2200 hours later.

Just to satisfy my own curiosity, what answer were you looking for that would convince you your engine is operating normally ?


I will work on injectors and go from there.
If you do "work" on the injectors, I think that's about the end of your boating for this year.
 
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Jun 15, 2023
13
Catalina 28 Santa Barbara
Ralph,

Thanks for the reply. I don't want to touch change anything that is working correctly, the problem is I don't know what is a normal vibration to know if there's something wrong. We had previously boat with a 5411 and did not shake the boat as much as this one. I am running at almost 1300 rpm, manual says idle range 800 to 1100. I have attached a couple of video that might help to see what I am seeing. Again I just want to go sailing, not replacing injectors. Also, I don't know if can be seen on video but the rpm gauge seems to go up and down a little bit during idle.
These are the videos they did not come up so good. Can really show the vibration and what I am listening.
Videos
 
Jan 11, 2014
11,883
Sabre 362 113 Fair Haven, NY
The idle seems high but the vibration does not seem all that bad. It sounds like a 3 cylinder diesel, which it is.
 
Nov 6, 2006
9,952
Hunter 34 Mandeville Louisiana
That high idle may cause gearbox problems because of the shock of going into gear (initial shift ) The frame rate of the video shows the amplitude of the vibration, but makes the frequency appear very low.. (think wheels on video appearing to be in slow-mo) .. That said, the amplitude does not look too bad for a three cylinder engine. I would turn the idle down a bit to give the gearbox a break (no pun intended?) when going into gear.
 
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Jan 4, 2006
6,735
Hunter 310 West Vancouver, B.C.
I am running at almost 1300 rpm,
I presume that's your set idle at 1300. Could you shoot a video at the recommended 800 RPM so we can get an idea of what the vibration is at that RPM ? 1300 is a pretty high RPM at which to shift. Adds years to the age of the clutch. I know my 2GM20F is rough at 800 RPM but that's just the nature of these small beasts.

Also, I don't know if can be seen on video but the rpm gauge seems to go up and down a little bit during idle.
That too is something which happens if you reeeeeeeeealy look for it. I really hate to say this but you "may" have a case of OCD which is worse than mine if that's at all possible. Mine is so bad I refer to it as CDO just to keep the letters in alphabetical order.

We had previously boat with a 5411 and did not shake the boat as much as this one.
I think that's a tough observation to make when you can't compare both engines running side by side. Maybe your video at 800 RPM with the engine rattling off the mounts will tell a different story.

Just for now, when you have a few minutes to spare, try setting your valve clearances spot on. It's no cost, relatively easy, and fairly quick. If you haven't done it before, download the manual and follow religiously. I suppose if one cylinder were way off, it could cause vibration.

Other than that, unless the videos lie, I would say your engine is running perfectly at 1300 RPM.
 
Jul 8, 2012
134
Catalina 36 MKII North East
Remove the fiberglass panel in the head and then run the engine. Check if the water muffler is tightly attached to the hull. If those screws are loose or the exhaust where the water inlet is attached touches the bulkhead or fiberglass panel it will rattle your teeth out. If you still have a hard hose to the water muffler consider changing to silicon hose to reduce vibration to the hull
The idle is too high. Lower it so both tachs are reading around 800. Repeatedly shifting at 1300 will kill your transmission.
If vibration is still a concern find a diesel shop to inspect and service the injectors. They usually turn them around in a few days.
A bad injector doesn't "get better" at higher rpm but the effects get hidden as speed and momentum grows.
 
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Jun 15, 2023
13
Catalina 28 Santa Barbara
Thanks for all the replies. I had to take her out of the water for bottom paint. I will do another video at 800 rpm once get it. I did check the water muffler because I had a little water in the head after running engine at the beginning. My thought was cracked muffler, but luckily was not the case. All screws are secure. I secure all the hoses and clean up the vented loop valve and the water leaking stopped. I did get a silicone hose from catalina direct to change where the exhaust hard pipe transition to water muffler but have not done that yet. I will change idle to not damage transmission as soon as I get it.

I will look into valve clearances as well. I will report back stay tune.

Ralph, I might be an ocd case like you mention....lol. Thanks again everyone for spend the time responding the thread.
 
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Aug 18, 2024
1
Ranger 29 Seattle
Same thing I have at 1200 RPM with a 1999 Yanmar 2GM20F. The engine is vibrating at a specific frequency called a harmonic. Most engines have this. Just stay away from that RPM as best you can and you won't have a vibration problem.
In general engines with fewer cylinders run rougher at idle than those with more cylinders. A 1 cylinder 4-stroke with fire every other rotation, a 2 cylinder engine will have a cylinder fire on every rotation, a 4 cylinder will have a cylinder fire every half rotation. At high RPMs the it doesn't make much difference because the cylinders are firing so quickly, at low RPMs the time lag between cylinder firings is longer so the vibration is more pronounced.
Hi, this is my first posting on SailboatOwners. The thread is a year old but there are some things about engine vibration that are timeless and maybe worth thinking about. Sure, there can be many factors that can affect vibration, but I think the sailors above have stated the bottom line pretty well - get above the harmonic frequency if you want quiet. The basic issue has to to with mass and thrust distribution in the rotating parts of the engine. The springiness of the mounting means there will be a resonant speed for the whole system, like any mass-on-spring system. Below the resonance, the engine's movements are largely transferred through the mounts to the rest of the boat, your rattling teeth, etc. Above the resonance the engine is sort of floating on the mounts and much less motion is transferred to the boat. In a 3-cylinder engine the thrust or impulse that occurs when each cylinder fires is probably the dominant factor although mass imbalance or shaft misalignment may contribute.