Sudden shift to lower RPM

[Remrio]

I am hoping someone here might be able to provide some light on my issue.

I have a 2008 Bav 46 Cruiser with a D2-75 engine with a turbo.  The turbo was completely rebuilt in Nov. 2019 as condition of sale to me.  I sailed her 30 miles from Annapolis to my marina in Rose Haven, Maryland after taking delivery.  She stayed in water over the very, very mild winter.  No freezing, no issues.

I sailed her once only since then and only under engine just two weeks ago. Everything started as normal.  An hour into my sail, I increased speed to just below the upper end of her ability.  She didn't quite respond as expected.  After approximately two minutes, she automatically and without any action by me, shifted down to a lower rpm while the shifter stayed at the higher rpm level I initially placed it at. I repeated this process three or four times before leaving her be so as not to push my luck.

This is an extremely clean and well maintained engine with less than 500 hours on it. Any ideas of what the issue could be?  Any insight would be appreciated.

[sy_Anniina]

To my experience and based on what I have read around, by far The most common cause of all rpm problems is fuel problems. Have you changed / inspected filters, and the possibility of dirt or diesel bug on the tank?

BR,
Tommi
s/y Anniina

[elias]

I agree with Tommi,
The most common fault around my sailing neighbourhood is fuel issues after winter rest , check the filter, eye inspection through the glass pot of fuel water separator ( mud/slime )or open the tank lid and inspect , even if you change the filter and the tank is  dirty the problem will repeat after healing while sailing. Else is something more complex than dirt

[SorinCT]

As a first start by checking all linkages for the fuel pump levers, maybe also verify throttle arm has no slip on cable jackets. If control side is ok next suspect would be fuel delivery (contaminants or air) which would determine faulty operation of fuel injectors. A sudden drop of rpm with no smoke or noise would not tipically happen due to a faulty turbo. You could have a look at the air intake though....

[PEA-JAY]

Check you fuel tank air vent before you touch anything else! I think you probably will find it blocked. Otherwise next time it happens try opening your filler cap. Raph.

[IslandAlchemy]

Change the primary fuel filter

[Domi409]

Fuel is very common and it gets worse when the fuel tank is not topped up completely during winter as condensation will easy bug growth and water at the bottom of the tank. Checking fuel filter and pre filter will help sorting the issue.

[Klausen]

Diesel-Bug?!!!

[sy_Anniina]

Diesel-bug = microbiological growth (algae, bacteria) on modern diesel bio-components when water and air are present. This growth can appear as slime or cloudiness and will block easily filters and on severe cases even fuel lines / valve / transfer pump.

BR,

Tommi
s/y Anniina

[Craig]

Have a look at the following You Tube clip.

This explains most of the diesel bug problems I have seen in Bavarias. Your Off/On switch is probably partially blocked.
https://www.youtube.com/watch?v=GS86BsbisNc&t=201s

Your filters are probably OK.

Craig
"Shirley Valentine"
Gold Coast
Australia

[Lyra]

Although the suggestions above concerning fuel supply are logical, my experience is that in such cases the engine either turns off completely after some time or starts to rev up and down arbitrarily. I do not remember a case where it  stayed at a constant lower RPM.
Another option, although it should have happened immediately when trying to rev up and not after some time, is a blocked exhaust mixing elbow - after 500 hours this is something that may happen if the elbow was never cleaned. A common sign for this is when you start the engine you see a gray circle of soot on the water surface at the exhaust outlet.

[diverphil]

hi, when you say tried it a few times, does that mean lower revs for a while with the throttle moved to a lower setting, then its ok , then when you increase revs its ok for a few minutes but then dies down again on its own without you moving the lever, if that's the case then I would go for fuel starvation as the cause. at any of the points it could block up , pick up pipe, fuel tap, pre filter, main filter.
are there any other signs of things not quite right with the engine, i.e. running hotter than normal as this would point to the exhaust elbow blocked causing lack of air going into the engine that would also show in the colour of exhaust gas. black smoke due to over fuelling as the air cant get in if the exhaust cant get out.