Cummins all of a sudden wont hit rated RPM

[zoomiej]

For whatever reason, my 396 running 370 Cummins will not hit its rated rpm. Both engines are topping out at ~2800 rpm (should be 3300) There was growth on the running gear and bottom, which I had a diver clean. There is no increase in the boat weight and even ran today with 1/4 tank of fuel.

Is there anything from a turbo charge or fuel I should be looking for? Two engines, would one working at 100% pull the other up to rated RPM?

Looking for trouble shooting ideas please

Thanks

[Viper]

When was the last time a fuel filter service was done? If you have Racor primaries, do the bowls look clear or dirty? If they're dirty, they should be dismantled and cleaned, and filter elements replaced along with the secondaries on the engines. Are your intake filters clean? Common causes for that symptom. Is there any smoke that's not normally there?

[zoomiej]

Thanks Viper,
Bowls are clean but I should change the filters. No smoke whatsoever, but will take a look at the air intake side as well. Thanks for the note.

[buster53]

Even though you had a diver come by, the bottom of your boat is not completely clean. For that, you need a short haul to completely clean the bottom.
My 356 will hit 35 mph with a clean bottom. 6-7 weeks ago, my top speed had dropped to 20 mph.
Had a diver come by and after he did his job, I got up to 32 mph.
I'm going to haul the boat for the winter and fully expect to hit 35 when I splash next spring with a clean bottom and fresh coat of bottom paint including Pettit Zinc Coat paint on my running gear.

[bud37]

From my limited knowledge of marine diesels, it may just be that one engine is creating the problem, there by holding the other back.......now that said, you did say this was an all of a sudden occurrence.....so consider anything just recently done on board.

I can say, if I had a marine diesel powered boat, the first thing , I would join Boatdiesel forums.

[zoomiej]

All good advice and thanks. Had the boat hauled and bottom done, fuel filters changed, turbo charger filters cleaned and still 400 rpm short. I suspect it is the cumulative weight of the boat from last year unless, one engine is indeed holding the other back. Thanks again for your input.

[PJHoffnet]

Is the shortfall only when under load (in-gear) or do you see the same thing just sitting at the dock and not in-gear? Don't run them at WOT for too long at the pier, and only after letting them warm up.

You might find the Seaboard Marine Cummins forums a great place to find answers to your engine questions.
https://www.sbmar.com/community/

[Zoomie]

It was only under load. Bottom cleaned, new filters, turbo charger filters cleaned and now just shy if rated. I suspect it is not down to the fact that I have too much weight on the boat. (Living aboard).

Thanks for all of the responses.

Cheers

[Zoomie]

It was only under load. Bottom cleaned, new filters, turbo charger filters cleaned and now just shy if rated. I suspect it is not down to the fact that I have too much weight on the boat. (Living aboard).

Thanks for all of the responses.

Cheers

[rmidgett]

Newbie question: how does one engine "hold the other engine back" from making full power in this situation? Assuming no synchronizer between the two engines.

Source of the post From my limited knowledge of marine diesels, it may just be that one engine is creating the problem, there by holding the other back.......now that said, you did say this was an all of a sudden occurrence.....so consider anything just recently done on board.

I can say, if I had a marine diesel powered boat, the first thing , I would join Boatdiesel forums.