Engine vibrates quite violently at low revs.

[Richard10002]

Moody 44 + Volvo Penta MD22L

last year I put the wrong oil in the gearbox and, after a longish trip, there was a vibration at low/idle revs and possibly a grinding going in and out of gear - so I identified the right oil and changed it, which solved the problem.

A similar thing started to happen during the recent long trips, so I suspected, and changed, the gearbox oil, with no effect.

The vibration is quite violent at low revs, so I have increased the idle speed to 900 revs, (it reduces to 800 when i put it in gear, and the vibration starts at around 700 revs, so I could slow the idle by another 50 rpm I guess.

There has always been an irritating vibration at low/idle revs, and I had intended to increase the idle speed to the point where it just stopped, which I have effectively just done but:

Could the more serious vibration at only low revs be a symptom of a more serious problem?

It's purring away nicely and quietly at 900 revs as I type, so there is no issue at higher revs.

Many Thanks

Richard

 

[VicS]

This is just happening at idle speeds out of gear?

Clapped out engine mountings?

Maybe not firing on one cylinder at low revs due to injector, pump or compression problem.

700 rpm sounds slow to me. What does the w/s manual say the idle speed should be?

 

[Searush]

I thought Volvo MD's were designed to make your teeth rattle at tick over. My former MD2B was a pain at tick-over, cups would jump off the cabin table (OK slight exageration). A recent change to a 3cyl Yanmar has transformed our low revs motoring.

 

[Talulah]

Sounds like the pressure plate. How many hours has the engine done? If it is the pressure plate the heavy vibration will later on get accompanied by some scraping noises.
If the pressure plate on this engine is similar to others the pressure plate itself has a group of rubber pads glued to it. After a while these pads start to get ripped off/shredded.
The replacement pressure plates were far more sunstantial.
(This is a job that can be done by a competent diy mechanic. Took me a day to do one)

 

[charles_reed]

Idle speed

according to the manual is 775-850 r pm.

Suspect you've set the idle too low.

 

[jimbaerselman]

Slow speed vibration (quite teeth rattling at times) is caused when the natural frequency of vibration of engine mounts (ie, their 'springiness') matches a disturbance frequency.

Disturbance frequencies are caused by:

1. Gearbox output rpm, matched by very slight engine mis-alignment (but not if you have a UJ).

2. Engine rpm

3. Depending on the number of cylinders, the firing frequency of one of the cylinders.

4. Whip introduced by softening flexible drives between engine and gearbox, or gearbox and prop shaft (if either or both are applicable)

Softened engine mounts usually lower the vibration frequency, and at the same time introduce some engine mis-alignment. So this is often the first culprit.

Excessively low engine rpm is the next culprit, and raising this, as you've discovered, can stop vibration. But it may hide another cause.

With these useless facts you may be able to track down whether there's a cause other than low rpm . . .

 

[Richard10002]

Vic - happens at idle speed of about 700 and below, both in and out of gear.

I dont have a w/s manual but the owners manual says one laying up job should be done at a "fast idle" of 900-1100rpm.

Charles - that's reassuring, and is the answer i'm going to go with for the time being <g>. I dont think the idle speed has ever been changed. The owners manual says changing it voids the warranty. The adjusting screw is painted completely green, and I couldnt move the locking nut - so I put a nylock nut on the business end and screwed it in until the idle was at 900.

I might take another 50 rpm off today, but it sounds fine at 900. What harm can a slightly fast idle do?

I hope it's as simple as perhaps the adjuster or some linkage had worn over the 13 years - dont like the sound of pressure plates or engine mountings - not until winter anyway.

Thanks to all for the help so far.

Richard

 

[Richard10002]

Jim,

I dont think I have a UJ, or any flexible couplings. The prop has a circular plate which is bolted to a similar plate on the gearbox.

I guess I'll see how it goes, keep an eye on the engine mountings, and perhaps change them in Malta - is this a DIY job, or is it bigger than that?

Many Thanks

Richard

 

[wotayottie]

[ QUOTE ]

I might take another 50 rpm off today, but it sounds fine at 900. What harm can a slightly fast idle do?



[/ QUOTE ]

The idle speed chosen is a compromise between slow enough not to b**er up the clutch / make engangement jerky and fast enough to avoid engine vibes. Thats why they make the gttee comment. So I wouldnt go too far away from volvo's recommenmded speed.

You commented in an earlier post about mount bolts coming loose. Vibes suggest a problem with mounts so it will be worth your while carefully checking. Your engine is bigger than mine but when I checked I found one delaminated mount and one sheered stud - the latter being impossible to see (not just by me but also a chap who worked for Petter etc and has been around engines all his life) until I levered up the engine with a crowbar and half the mount stayed on the engine bearer!

 

[VicS]

[ QUOTE ]
700 and below, both in and out of gear

[/ QUOTE ] If it happens out of gear then that rules out any problems with couplings or misalignment.
[ QUOTE ]
engine mountings, and perhaps change them in Malta - is this a DIY job, or is it bigger than that?

[/ QUOTE ] The tricky bit, especially if you have not done anything like it before, will be aligning the engine and propshaft once the new mountings are fitted.

 

[Richard10002]

[ QUOTE ]
[ QUOTE ]
700 and below, both in and out of gear

[/ QUOTE ] If it happens out of gear then that rules out any problems with couplings or misalignment.

[/ QUOTE ]

Makes sense..... At least there is some relief <g>


[ QUOTE ]
[ QUOTE ]
engine mountings, and perhaps change them in Malta - is this a DIY job, or is it bigger than that?

[/ QUOTE ] The tricky bit, especially if you have not done anything like it before, will be aligning the engine and propshaft once the new mountings are fitted.

[/ QUOTE ]

presumably, you have to lift the whole engine to put them in, (I'm thinking 4 hydraulic jacks and maybe a couple of thick planks).

How do the experts align it - presumably there are some measurements before removal then, when it's close, spin the prop by hand and see if it's biased either way. If so, gradual adjustment of the mounts?

Cant be rocket science, just needs to be right.

Anyway, I've got a bit of time to research it and, if I dont feel comfortable, i'll get the yard to do it.

Thanks to everyone for the help.

 

[Searush]

Should be able to lift the engine using planks & wedges or levers. Check them individually by lifting as mentioned earlier to see if they are split.

Key thing is to make sure alignment is not lost.

 

[boatmike]

Forget all the rocket science. 700 RPM is too slow try 800/850. Quite normal for these engines. Also expect a little "clunk" when put in gear. Quite normal. Stop worrying and go sailing!

 

[Richard10002]

[ QUOTE ]
Quite normal. Stop worrying and go sailing!

[/ QUOTE ]

The pragmatist appears and puts it in a nutshell! <g>

That's what I've done. In fact I'd kind of done it before asking the question. Just wanted to make sure I wasnt hiding something serious.

It's just that i have a slight concern regarding the mountings, so no harm in being prepared.

Thanks for the encouragement!

 

[Gunfleet]

I've never understood why aligning is supposed to be so difficult. If you can find somewhere to mount a magnetic dial gauge (cheap now on ebay) and turn the shaft you'll know which way to shim the mounts. I'm sure setting a shaft up from scratch needs real skill... but just replacing the engine mounts?

 

[VicS]

[ QUOTE ]
Key thing is to make sure alignment is not lost.


[/ QUOTE ] True but it would be very sensible to check and if necessary adjust the alignment after changing the mounts.

 

[wotayottie]

Its almost inevitable that if you replace the mounts you will need to re-align.

There are 2 stages to alignment. In the first you are aiming to have the shaft running near central in the stern tube. Over time, as the mounts have softened, the shaft will have dropped. Since you go forward much more than reverse it will also have moved to one side, usually to stbd but this depends on prop rotation. If you are out of the water then a simple spacer to fit inside the stern tube and hold the shaft central will help. Then all you need to do is to adjust the mount heights and the engine lateral position to the point where the shaft, when slid forward, slides smoothly into the engine coupling. Part of this stage will be achieved simply by fitting the new mounts and replacing the soggy ones but since the engine bearers are rarely equal height, then the engine mounts are rarely adjusted the same, side to side.

When you have done this, you need to check the angular alignment which you do with the coupling now attached to the shaft. With a space of maybe 20 thou between the shaft coupling flange and the engine flange, you measure the spacing at 12 o'clock, 3 , 6 and 9. All need to be the same or at least within a couple of thou. This stage is best done in the water since the shape of a hull changed when the boat is out of the water. It is an indication of a bent shaft if you rotate the shaft (but not the gearbox) and find that a shaft which was aligned goes out of alignment.

The whole process might take and hour or two depending as much as anything on how easily your engine moves on its bearers when the mounts are slackened. But as you rightly say, its basically a simple process that anyone can do with care. If it werent, I couldnt do it!

 

[jimbaerselman]

You'll find a very informative site on mountings and alignment at: http://www.yachtsurvey.com/alignment.htm

It should answer most of your queries, but don't be too put off about all his dire warning about sinking boats . . .