Engine Vibrating Violently when Shifting to Neutral

[Heikki]

What might me the reason for our engine (Volvo Penta MD22L-A,4cyl, 50hp, year 1995, 4600 hours) to vibrate violently when shifting to neutral after running at normal cruising speed? The engine is coupled with normal/simple transmission and solid axle with fixed propeller.

This is a problem that appeared suddenly two days ago and has now happened three times - every time I come to marina. It vibrates so much, I can both well - way too well - feel and hear it on the cockpit. To get the shaking off, I need to switch between forward or reverse and neural for couple of times.

Few things I have already checked:
-When I start the engine, it runs very smoothly on idle
-There is nothing wrong, no noise or vibration, when cruising at normal speed
-The "cushions" (sorry for not knowing the correct English term) between engine and the steel hull seem to be fine
-The propeller is clean and undamaged
-The engine and the axle are perfectly in line
-Fuel filters are clean
-The problem does not appear if I run the engine idle on or low speed when on mooring
-When testing on mooring with low engine speeds, there is a slight shake when switching to reverse, but no shaking/vibration when switching back to neutral or forward

Could it be a loose fly-wheel? That is something I have been unable to check because it sits in a solid housing between the engine and transmission.

We have just started our two weeks family vacation. As the next step, we should go through a channel from an inland lake to the Baltic Sea. The other end of the channel is on the Russian side of the border with a 30 mile sea passage before back at domestic waters. I would hate to stop our trip here, however, I would hate even more the get in trouble with engine when on Russian waters. Today is a public holiday and tomorrow is Sunday, so no professional help is available locally. Any advice how to fix the problem and how to proceed are greatly appreciated.

[James S]

The "cushions" are the motor mounts, and although they may look fine they may not be fine, depending on how they are constructed.
I suspect that one or more of them may be broken or loose. This may only be visible when you shift to neutral and the torque changes.
I suggest having a good look at them while shifting.

[Heikki]

Originally Posted by James S

I suspect that one or more of them may be broken or loose. This may only be visible when you shift to neutral and the torque changes.
I suggest having a good look at them while shifting.

Thank your for your reply, James. What I did not state clearly in my original message, is that the vibration stays on for extended period of time. It is just not a single moment, or something for 2-3 seconds. I do not know, how long it would last - this far I've stopped the engine or increased throttle within about 15-30 seconds to stop it.

Despite of a public holiday here, I was able to reach a one Volvo repairman located on the other end of the country. According to his opinion, it would not be flywheel - it would shake and rattle regardless of the revs. Sure enough, he could not say anything for sure, however, he supposed the fault might be in something that could be in English a "dry clutch plate" or "coupling plate". Something loose or broken there? Any experience? Any ideas?

[boatman61]

Sounds to me like maybe a mixture problem... check your filters fuel and air and maybe your injectors for deposits... or just up your tickover speed a tad...
But I'm usually wrong...

[Unicorn Dreams]

Next time after shifting into neutral, slowly shift into reverse and see if tranny is disengaging totally.

[sabray]

I like james idea engine mount. It only shakes in neutral after being run so engine is warm. What are the tons at idle in neutal. When Tia are in gear the good mounts are tightened so they hold the engine in neutral they slack and the engine is free. I can't see a damper plate failure doing this It will still be turning in neutral. It's seems it is still engaged when in gear. I'd rather have it be a mount. I winder could you strap or lash the mounts. Or just pull them one at a time to check them. Sounds like your not going anywhere today. What better thing to do on holiday.

[Seahunter]

If the engine runs fine, no black or white smoke, hot or cold; it's probable a broken motor mount.

[James S]

I guess it could have something to do with the damper plate...seems a long shot though.
Some damper plates incorporate rubber blocks...perhaps when the transmision goes from having a heavy load, in gear, to having a light load, in neutral, the blocks orientation changes enough to cause a vibration...the blocks do wear and can be damaged causing them to loss mass.
??

[DeepFrz]

A long shot, but, check the throttle linkage at your governor to see if it has anything to do with it. It seems strange that you have to shift it in and out of gear a few times before the engine smooths out. Another thing to check is if the prop shaft is turning when the engine is running rough at idle. Good luck.

[impi]

Have you checked transmission mounts / bolts are tight ?

[impi]

Just as a matter if interest, I had a similar experience on my one yanmar - mountings looked fine but when removed I could see the engine mount bolt had sheared - replaced and smooth as a whistle. Try get a bar in under there and wiggle it around to see if they are tight

[jrd22]

I think it sounds like the transmission gears may not be fully disengaging for some reason, possibly worn shift linkage (internal or external). Watch the transmission when you change from forward to neutral and it is vibrating, you might be able to tell if it is actually going all the way into neutral. The fact that you have to shift several times with the engine at idle and then the vibration stops doesn't sound like a motor mount to me, although they can cause strange things to happen when one or more is broken.

[Heikki]

Thank you all for all the advices and ideas. I have - with a yacht club boat inspector - checked the mounts with a good strong bar. They appear to be ok, but sure enough we cannot be 100% sure.

The idea of the transmission not disengaging fully has also crossed my mind, however, I have to check the situation with the boat really running at normal speed - when I test this at marina berth with low revs, there seems to be nothing wrong.

I personally still suspect that damper plate as a possible cause. I have to take the transmission off today and see how the plate looks like. Volvo repairman suspected that because this all started suddenly without any prior warning or symptoms. If the vibration is due to motor mounts, that is unlikely to stop me in the middle of Russian waters. However, should the damper plate give up, then I will have no propulsion and sailing is strictly prohibited in the channel. I just have to check it and hope that I can tell if it is good or bad - I have never seen a one in my life before.

In addition to all you friendly people, who have given me your time and thoughts helping me out, I have had some luck with the marina surroundings here. The kids - I have five of them aging from 8 to 14 years on board - like the local playgrounds, swimming area, and indoors spa that is included to a quite reasonable marina fee. My wife is happy, too, so I just have to sort this out. Yesterday was raining all day, so I could really do nothing with all of them on board. Today they will head to the town and I can start to work - hoping to fix more things than I break or loose...

[pillum]

I'm with Boatman on this, I'd be looking at engine imbalanced operation as the load comes off. Could be worthwhile to have it tuned even if just to eliminate it as an issue. Remember the old engineering adage about process of elimination.

“Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.”

[cwyckham]

If it was a problem with alignment (which I have had) or with the mounts, I think it would show up at the dock. Sounds like either the transmission isn't disengaging all the way after it's been loaded or possibly the damper plate. There is a picture of the a good and bad plate here (scroll down to June 29th entry) on a Volvo engine (don't know if it's the same model). I had a bad plate on a Westerbeke (very different looking with springs instead of rubber). I think a bad plate would give a few bangs coming in and out of gear, but not a continuous rumbling, but that's just a guess. I'm still voting for transmission.

When you figure out what it really is, please let us know!