Vetus diesel shakes at idle

[Ryban]

The Vetus M4.15 on my boat idles (should be ~840RPM) pretty roughly, and shakes quite a bit, to the point where I think it's going to stall. If I bring it up to ~1000 RPM or so, it smooths out considerably. As a result, I've adjusted the set screw for the throttle so the engine idles a little higher until I can figure out what the problem is.

Where should I begin on debugging this? Should I just get an expert out to take a look?

[Bill O]

Is the rpm readout correct? Not sure on that motor if the tach is off the alternator. Did you change the alternator?

Other than that, did you check all the things listed in the trouble shooting section on pg. 56 ("rough idle") of the owners manual?

[Tortuga's Lie]

Check the engine mounts as well

[Lepke]

Originally Posted by Tortuga's Lie

Check the engine mounts as well

A bad motor mount can let a diesel shake.

[Searles]

First check accurately true idle speed ,set about 850 rpm ,check engine mounts if over 10 yrs old ,also service injectors ,at the same time check injection timing ,⛵️⚓️

[bil56]

yes, check:
- idle sped
- engine mounts
- alignment (load distributed)
- injectors and injection timing

[Ryban]

I'll work on checking these items. I just came back to the boat today after several months away and went to run the engine and it shook similarly at the higher set point (~1000 RPM), to the point that it sounded like it was going to stall, so I cranked it up a little. Once the engine warmed up, it was ok at the 1000 RPM set point. It still does vibrate a lot more between 1000 RPM and 1400 RPM though.

Here are two videos. The first at 1000 RPM (not the 870 RPM recommended int he manual, but you can still see that the engine is moving much more significantly -- even though the framerate of my camera weirdly synchronized with the movement, it's obviously shaking at a much higher rate than the video is showing.
https://www.youtube.com/shorts/bD9rpXPoJCE

The second is at 1400 RPM and this movement has ceased.
https://www.youtube.com/shorts/EHe7yXoQvyQ

[Pete7]

It would be interesting to see what 870 revs are like, once it has warmed up.

Do give the mounts a good check, probably need a pry bar to test them.

However, at 1000 revs I am not seeing a problem I would be truly worried about. Perhaps a more watch and wait approach.

Different engine, but we changed the mounts a few years back and it made a huge difference. A variety of wooden blocks of 4x4 and a crow bar enabled each corner to be lifted and supported in turn whilst the mounts were changed out.

Pete

[skipperpete]

Originally Posted by Ryban

I'll work on checking these items. I just came back to the boat today after several months away and went to run the engine and it shook similarly at the higher set point (~1000 RPM), to the point that it sounded like it was going to stall, so I cranked it up a little. Once the engine warmed up, it was ok at the 1000 RPM set point. It still does vibrate a lot more between 1000 RPM and 1400 RPM though.

Here are two videos. The first at 1000 RPM (not the 870 RPM recommended int he manual, but you can still see that the engine is moving much more significantly -- even though the framerate of my camera weirdly synchronized with the movement, it's obviously shaking at a much higher rate than the video is showing.
https://www.youtube.com/shorts/bD9rpXPoJCE

The second is at 1400 RPM and this movement has ceased.
https://www.youtube.com/shorts/EHe7yXoQvyQ

Are the videos taken with the engine running in gear or neutral? And is that the original alternator?

[Bill O]

Ryban,
My first approach would be to do the "simpler" things in the manual, especially since the section emphasis is "rough idle".
If that doesn't fix it, would then look at motor mounts, etc.

The other thing you need to ask yourself is, what changed? Did you recently change the fuel filter or run low on fuel? Fully bleed the engine to get all the air out. This is good practice as some motors need a systematic approach to bleeding the system. (if you haven't changed the secondary filter on the motor would do it before doing the systematic bleed)

If the rough idle is something new, any clues to what changed could be helpful to fix the issue.

[donradcliffe]

Some older engines idle rough when cold because one or more cylinders don't fire until they warm up.

If you Crack each injector nut individually while ir is rough, the cylinder that doesn't make things worse is the one that is misfiring

[Ryban]

To answer the above, those videos were taken without any load on the shaft. Before I go playing with anything else, I did manage to get a direct RPM measurement.

Update:
I decided to start with what was easy and check the RPM with several different "strobe light" RPM apps you can download that just flash your phone's flash LED at a specified rate. It will have to do until I get my hands on a tach gun.

Using these tools, I've found that my RPM gauge is reading 300-400 RPM high. For example, in the 1400RPM video above, the crankshaft seems to be turning at closer to 1050-1100RPM. At the 1100 RPM on the gauge, about 910 RPM was measured.

I guess the next question to address is why is the gauge reading high.

[skipperpete]

Good work Ryban. I can’t remember seeing a tach sender on the Vetus engines so it’s likely that the alternator supplies the pulse, is it the original? Has the tachometer head got an adjusting screw on the back?

[deblen]

https://www.youtube.com/watch?v=BoASpitO7GQ

You said you have been away from boat for months.
Has your fuel lost some cetane? Add 10% kerosene.
Cheers/Len

[Ryban]

That's a very clever and simple way to measure RPM, never thought about that.

Fuel is probably not that old over all, as I've topped it up with fuel a few times, but there is probably some old fuel in there still. I just added some lucas oil injector cleaner, even though it probably won't do anything, but I don't really have any evidence that there are issues with the injectors.

The alternator is certainly not the original, but I also don't know when it was last replaced. I'll check to see if there is a tach head anywhere with an adjustment screw.