RPM drop

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SeaDogAK

Senior Member
Joined
Aug 9, 2021
Messages
356
Vessel Name
Sea Dog
Vessel Make
1991 DeFever 49 RPH
Coming back from the first trip of the year (about 3 hours running each direction) I had an unexplained drop in RPM on the starboard engine. We normally cruise at about 8 knots, 1800 rpm on twin Perkins T6.3544 engines.

About 2 hours out on the return trip, I noticed rpm’s slowly dropping on the starboard engine. I advanced the throttle which brought rpm’s up some but they continued dropping, eventually settling even with more throttle at 1600 rpm. The engine sounded fine, and temperature and oil pressure were normal, so I ran the rest of the way home at 1600 on that engine. My guess (hope?) was maybe we had dirty fuel — I’m still using up last years fuel so it’s a year old (though I polished it a few weeks ago) - and I thought maybe a fuel filter clogged (though all filters are new this spring).

I just checked the Racor and it’s clean as a whistle. I also just checked tachometer calibration and the tach is dead on (brand new tachs this year).

The engine starts fine, sounds fine, and will rev to 1800 fine, just with a bit more throttle.

My throttles have always been uneven - I would always need a bit more throttle on the port engine to get the same RPM’s. So now it takes about the same throttle on both engines to synchronize the RPM’s at 1800.

Do I just shrug and say it’s just one of those boat mysteries, or is there something else I should be doing to figure out why it’s doing this?

I should note both engines have always smoked a fair amount (white smoke), and I’ve thought the injectors might be dirty or worn out, so it occurs to me that this might be an injector issue.
 
Have you ever run them up to WOT in the past to see what RPM and boat speed you achieve? If you have a reference for that I'd be tempted to do that test now and see if you get normal results or if the stbd engine seems to be down on power and can't reach normal RPM.

It's also possible you picked up weeds or debris on the stbd prop causing an increase in engine load. If that's the case, it may be gone now and the problem won't be reproducible.
 
I don’t have a reference for WOT rpm. I do expect to have a diver on the boat in the next couple of weeks so I’ll be able to confirm that there’s nothing on the prop.
 
A few observations:

It's unlikely fuel or filters. If you could initially gain back the RPM, that's counter to a flow restriction in a filter. While I'm not familiar with your particular engine model, most diesels in boats have an injection setup where you're selecting engine RPM with throttle position, and the injection pump is regulating injection flow to maintain the RPM. When driving into waves, the extra varying load requires the pump to vary the injection to maintain RPM. This fluctuation can be seen in the slight variation of the RPM. If the fuel was poor, the injector pump would simply increase flow to maintain RPM.

If everything else is normal and nothing is on the prop, I'm thinking an injector pump problem where RPM regulates injection dose. Not being familiar with the engine, that's only a uneducated guess.

Ted
 
Changing fuel filters is quick and easy so I would start there but I doubt that is your issue. Next I would dive the running gear, won’t be surprised if you have a bunch of polypropylene wrapped around the shaft. If those two don’t solve the issue then injector pump becomes the main suspect.
 
Thank you for the ideas. I’ll check the shaft and prop next.
 
Did the throttle lever remain in its original pre rpm drop position or did it drop back?
 
For those that may know, why just an injection pump problem and not one injector, or injector line from pump.
Recently changed one injector that was missing like a bad spark plug.
 
Did the throttle lever remain in its original pre rpm drop position or did it drop back?
It did not drop back. In order to try to maintain RPM’s, I advanced the throttle farther than it would normally go.
 
For those that may know, why just an injection pump problem and not one injector, or injector line from pump.
Recently changed one injector that was missing like a bad spark plug.
If you have a bad injector, usually there an audible change in the engine sound, like it's missing on one cylinder, especially at idle.

Ted
 
Two things. First when was your on engine fuel filter last changed? Those pesky “0” rings can allow an air leak. Secondly, an air leak in the fuel system in front of the on engine all the way back to the fuel manifold may be lurking. Some clear tubing after the Racor is a good way to see air bubbles.

I’ve experienced fuel system air leaks with engine surging occurring. Lastly - injection pump, look for any sign of fuel leakage at the pump.
 
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