Engine tone change for a few seconds

[Beneteau381]

My Perkins M30 runs fine, but every now and then it changes tone for a few seconds, the revs don't drop but it just sounds like they do then it sounds normal again. I have changed the filters and made sure there is no air getting in, also it's clean fuel
It might not happen all day.
I am at a loss

Ive said it many times, the throttle sets the revs, when a load comes on such as with the boat hitting a wave, the governor opens the pump rack and injects more fuel, the note of the engine deepens as it burns it to produce more power to deal with the load. Sois anything happening to make the governor react?

 

[black mercury]

Is it possible the engine is running perfectly but your hearing could be faulty?

 

[LiftyK]

Does the change in note sound ‘thinner’ with less grunt and more of a hollow ting? If so, it might be crud or water in your diesel tank. I had this experience during rough weather. Were you bouncing around a bit when the note changed? I removed the tank and found much gunge in it. Now I add diesel treatment every time I fill.

 

[38mess]

Does the change in note sound ‘thinner’ with less grunt and more of a hollow ting? If so, it might be crud or water in your diesel tank. I had this experience during rough weather. Were you bouncing around a bit when the note changed? I removed the tank and found much gunge in it. Now I add diesel treatment every time I fill.

Yes it does sound like it's going to die but then roars back to life

 

[38mess]

Is it possible the engine is running perfectly but your hearing could be faulty?

More than likely

 

[[2574]]

Yes it does sound like it's going to die but then roars back to life

That’s not a “change in tone”, it now sounds much more like a fuel delivery issue.

 

[Adios]

Yes it does sound like it's going to die but then roars back to life

that's giving a very different impression than what you first described, disregard my previous advice not to worry about it!

 

[38mess]

that's giving a very different impression than what you first described, disregard my previous advice not to worry about it!

I don't think it's going to die, ignore that...it just doesn't sound as powerful for a few seconds. It's like someone is putting their hand over the air intake

 

[penfold]

Yes it does sound like it's going to die but then roars back to life

Not possible if the revs are not changing.

 

[Pye_End]

Can you tell us whether this happens in flat calm and/or bouncy conditions? Is it tack dependant (if motorsailing), or is it all the time? Is it coming down a wave with a following sea for example?

 

[38mess]

Can you tell us whether this happens in flat calm and/or bouncy conditions? Is it tack dependant (if motorsailing), or is it all the time? Is it coming down a wave with a following sea for example

It happens in most conditions. I can't really put my finger on it. It might not happen for hours on end, then twice in ten minutes

 

[chubby]

Do you have an electric fuel pump? if so an ageing pump may intermittently fail to keep up with demand

 

[srm]

Where are you when you notice the change in tone? I have noticed apparent changes of tone when down below that can be corrected by turning my head, something to do with the cabin's acoustics.

 

[RunAgroundHard]

My Perkins M30 runs fine, but every now and then it changes tone for a few seconds, the revs don't drop but it just sounds like they do then it sounds normal again. ... I am at a loss

Yes it does sound like it's going to die but then roars back to life

This explains a lot. You can't explain a symptom. How on earth do you get on with your doctor.

Thats why wet are at a loss to help.

Maybe explain it better, is it the first or second quoted symptom.

 

[Daydream believer]

Air intake filter coming lose? Sponge bit blocking the intake as it flaps around?

 

[38mess]

This explains a lot. You can't explain a symptom. How on earth do you get on with your doctor.

Thats why wet are at a loss to help.

Maybe explain it better, is it the first or second quoted symptom.

Thanks for your input

 

[RunAgroundHard]

Thanks for your input

Well, you want help and hopefully a solution, so you need to describe the problem in detail.

I have a Perkins 4236 (M90). After the fuel tanks were cleaned, complete drain down of fuel system, it used to give an audible tone drop, of about a second duration, a blip. To fast that when it was noticed, the rev counter could be checked. It happened about every 3-4 hours on a long delivery after the engine was recommissioned and system bread of air.

My fuel system does not have a return to tank over spill, it returns to the top of the engine fine fuel filter.

I believe that there may have been a small amount of air on the low pressure side that circulated from the final filter to the spill return line back to the filter.

It went away on its own accord and for few years now it has never returned, despite multiple fuel filter changes and bleeding operations.

It could have been water and not air from the tank cleaning job.

Check for water in the filter bowl, bleed the fuel system again. Or ignore it.

 

[Adios]

It happens in most conditions. I can't really put my finger on it. It might not happen for hours on end, then twice in ten minutes

in a river or at sea? Reason I ask is a river there can be little changes in flow where theres an eddy or water coming up off an obstruction on the river bed or whatever. changes to water will give a slight tone change but with pretty much same revs. Thinking about that because mine with the 4108 was on the Trent not at sea, might have been that