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Old 21-03-2018, 11:44   #1
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How to diagnose broken engine mounts?

Our starboard engine has developed a rhythmic "clunking" noise in forward gear that transforms into a horrendous "banging" in reverse. The vibration persists for a few seconds before diminishing when it is taken out of gear. The shaft is not bent, the alignment is spot on and I have switched props with the port side to eliminate the prop as the source. The only thing I can think of that would cause this is a broken engine mount(s). My question is, before I start tearing things out and ordering parts, how is the best way to confirm a broken engine mount? I have disconnected the shaft and tried levering up the mounts one by one but they seem to move about 1/4" and that is it. I tried the same process on the port engine and got the same result, leaving me with a tiny bit of doubt as to whether it is indeed the mounts, although I can't imagine what else could be causing the problem.

So my question is; how do you test for bad engine mounts, or is this even necessary given the symptoms?
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Old 21-03-2018, 12:05   #2
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Re: How to diagnose broken engine mounts?

Ergh, take a stick. A broomstick is a good starter.

Now break it over your leg. Can see see or feel the way it is now broken?

Same thing with an engine mount. It is a couple of steel bolts or studs embedded in a block of "elastomer". Either one of the metal parts is broken, or the elastomer is broken. Look at any two (or all four) mounts, and if any one of them seems cracked or split or distorted, that's broken.

Also look at the dirt around the engine mounts, on the engine and on the engine bed on the hull. Of there is a "disturbance", a place where the dirt has been pushed aside? That means the mount is moving. Either loose or broken.

It pretty much is that simple.

Now, I'd also throw in this: None of us changes engine mounts unless they're broken. No one really enjoys jacking up the engine and re-aligning the shaft. But, all the companies that make "rubber" parts like engine mounts? Say they only have a five-year life. Longer for silicone and more expensive elastomers. But they all age, get stiff as they get older and have more heat and vibration and ozone affecting them. So their recommendation is usually five years. If you've got the typical 20+ years....Yeah, it could be time.

But not hard to diagnose, just look for "What's wrong with this picture?" on any one of them.

The clunking and banging...Could be many other things. If you have a mechanics stethoscope (they're cheap) and can listen to the various parts of the engine block, sometimes it is faster to pinpoint what's clunking that way. If it is internal to the engine, you want to find out FAST to prevent more damage.
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Old 21-03-2018, 13:21   #3
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Re: How to diagnose broken engine mounts?

Quote:
Originally Posted by hellosailor View Post
Ergh, take a stick. A broomstick is a good starter.

Now break it over your leg. Can see see or feel the way it is now broken?

OK, I tried that and it didn't break. Now my leg hurts.

Same thing with an engine mount. It is a couple of steel bolts or studs embedded in a block of "elastomer". Either one of the metal parts is broken, or the elastomer is broken. Look at any two (or all four) mounts, and if any one of them seems cracked or split or distorted, that's broken.

That's just it. They all "look" fine, no broken metal and no distortions. The mounts are of the type where the elastomer is under a metal 'hood' to protect it from oils presumably. Unfortunately, you cannot see or feel the elastomer.

Also look at the dirt around the engine mounts, on the engine and on the engine bed on the hull. Of there is a "disturbance", a place where the dirt has been pushed aside? That means the mount is moving. Either loose or broken.

It pretty much is that simple.

Now, I'd also throw in this: None of us changes engine mounts unless they're broken. No one really enjoys jacking up the engine and re-aligning the shaft. But, all the companies that make "rubber" parts like engine mounts? Say they only have a five-year life. Longer for silicone and more expensive elastomers. But they all age, get stiff as they get older and have more heat and vibration and ozone affecting them. So their recommendation is usually five years. If you've got the typical 20+ years....Yeah, it could be time.

Mine are 8 years old with 1940 hours.

But not hard to diagnose, just look for "What's wrong with this picture?" on any one of them.

Unfortunately, they all look pretty good.

The clunking and banging...Could be many other things. If you have a mechanics stethoscope (they're cheap) and can listen to the various parts of the engine block, sometimes it is faster to pinpoint what's clunking that way. If it is internal to the engine, you want to find out FAST to prevent more damage.

I did this with my own version of the stethoscope; a long screwdriver jammed in my ear, and found nothing out of the ordinary.
Thanks for the detailed reply. What really makes me suspect the mounts is that the engine actually moves around in reverse, possibly due to the prop pulling the motor out of alignment. I have Kiwiprops and the thrust is quite a bit more in reverse than in forward. What has me confused is that there is no obvious damage to the mounts.
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Old 21-03-2018, 16:25   #4
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Re: How to diagnose broken engine mounts?

Quote:
Originally Posted by mikereed100 View Post
Thanks for the detailed reply. What really makes me suspect the mounts is that the engine actually moves around in reverse, possibly due to the prop pulling the motor out of alignment. I have Kiwiprops and the thrust is quite a bit more in reverse than in forward. What has me confused is that there is no obvious damage to the mounts.
Presumably this only happening on the stbd engine. If the port engine moves the same amount then this is not a good diagnostic approach.

Other way to confirm the mounts is to swap them over with the other engine - a hell of a lot of work but a good way to check.

The problem with all "rubber" products is that the external examination is almost always meaningless - a bit like looking for corrosion in SS, the bad stuff is inside out of view.
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Old 22-03-2018, 09:00   #5
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Re: How to diagnose broken engine mounts?

I had (and to some degree still have) a similar problem, although it's just a single 27HP Yanmar. These engines have very soft mounts so you can tug the engine around even if your mounts are not broken. I could not determine if they were defective, but they were rusty, so I decided to replace them. Three of four were "half broken", meaning that the rubber on one end had become detached from the steel portions. Gravity was the main force holding the engine in place.

I replaced the mounts, and things are better, but I used stiffer mounts and the engine sound is dramatically different, sounding almost as if the exhaust is disconnected. Very "huffy" compared to before.

Oh, and the vibration that I sought to eliminate when putting the boat in forward gear is still there, so I think I have a vibration damper problem, not an engine mount problem (but I needed to replace them anyway...).

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Old 22-03-2018, 12:42   #6
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Re: How to diagnose broken engine mounts?

Not to be a downer, but I had a similar issue with a Volvo MD3B. Turned out the transmission was shot. Keep us posted.
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Old 22-03-2018, 18:59   #7
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Re: How to diagnose broken engine mounts?

Forward, reverse, forward reverse, ad nauseum. You watch, someone else shifts, using engine rpm and boat speed to really torque the shafts, props and engine. Have them be aggressive so you can clearly see if there's a problem. If engine is rocking back and forth, or shaft is loose in coupler, you'll see it. Hit it hard.
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Old 22-03-2018, 19:21   #8
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Re: How to diagnose broken engine mounts?

Why not just pull them one at a time and inspect?
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Old 22-03-2018, 19:42   #9
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Re: How to diagnose broken engine mounts?

To test the mounts you need to pry the engine upwards somehow. Then you can see if the rubber is connected an how much flex.
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Old 22-03-2018, 20:16   #10
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Re: How to diagnose broken engine mounts?

What do they use to glue the elastomer to the metal plates?

If one has started to separate, can you use 5200 or something similar in the gap to glue it down again for a while?
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Old 23-03-2018, 02:45   #11
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Re: How to diagnose broken engine mounts?

Quote:
Originally Posted by SeaSon View Post
Forward, reverse, forward reverse, ad nauseum. You watch, someone else shifts, using engine rpm and boat speed to really torque the shafts, props and engine. Have them be aggressive so you can clearly see if there's a problem. If engine is rocking back and forth, or shaft is loose in coupler, you'll see it. Hit it hard.
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Originally Posted by Thumbs Up View Post
To test the mounts you need to pry the engine upwards somehow. Then you can see if the rubber is connected an how much flex.
All this hitting it hard and prying sounds like a good way to break something esle to me.

If you carefully block up the engine, you should easily be able to remove mounts one at a time for inspection without screwing up alignment.
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Old 23-03-2018, 05:39   #12
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Re: How to diagnose broken engine mounts?

Couldn't it be the joint to the propeller shaft? It has torsion AND axial pressure, so it can turn loose....

And yes, shock absorbers do get old... Try to stay in a hot engine room with 1/4 of the engine mass trembling upon "you" (your thumps) :-)
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Old 30-03-2018, 10:36   #13
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Re: How to diagnose broken engine mounts?

Why are elastomer type dampers used? Does anyone know why cable-vibration isolaters are not employed? They are virtually indestructible, available in marine and MIL spec and will outlast any rubber based isolaters. They also start at 5 daN so they could even be used for small fragile equipment.
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