RPM Fluctuation

vanburen

New Member
Feb 18, 2008
324
Annapolis / Alexandria
Boat Info
420 DB
Engines
Cummins 480 CE
Going to, and returning from BIH we had about a 100-200 RPM fluctuation in the starboard engine. Occurred from 2200 RPM dropping to about 2000. I had the sync running and the port engine didn't fluctuate.

Before heading home, I changed the RACOR element - it was blackish but not terrible, although I don't have much basis for comparison. Regardless, it didn't help - an hour into the trip back the RPM started dropping again. It would drop for a few seconds, and then ramp back up.

Any idea what could be causing this? Many thanks in advance.
 
ED,
I had this same issue for a year.... Cummins couldnt find it as it would run fine when they ran the boat at the dock. they hooked up all there lap tops ran tests they found nothing ... Long story short, I had a bad fuel injector. LIKE i said took them year to find it... I shot video of the smart craft with the engine sync on as they didnt believe me ... Eventually it got worse and worse

PS it was my stb motor as well

Rob
 
Last edited:
Rob,
Thanks again! Hopefully that will help "accelerate" the diagnosis and repair. The first time the RPM dropped was really disconcerting - after that I got used to it. We'll see what happens.
 
If the Racor was "blackish" and you are using 10 micron or larger filter trap sizes, I'd change the secondary filter before spending the money on a service call because dirty filters is the technician's "default answer" for fluctuating rpm's. Spend the $10 and get it out of the realm of possibility........who knows, we might get lucky and fix the problem.
 
Frank - thanks. To make sure I'm following, you are now recommending changing the engine fuel filter, not the Racor element, correct?

Also - do you always change the gasket/o-rings when changing Racor fuel filters? Is there an easy way to get that out?
 
Frank,
First thing they ( cummins ) did was change my filters. Didnt even start the fing boat. They left the filters with a note... sayingthey were DIRTY. 50 hrs on the racors and mains. Boat ran fine for a few hrs then acted up again... My marina changed the filters again... boat ran fine for a bit... U get used to a crappy running motor.... Thenwe went on a big trip.... boat ran great for 15 hrs.... then ran worse then it ever did for 5 in some pretty good snot.... I was not amused.... I went an entire season on a bad injector. . But ur logic makes sense change BOTH racor and main filter.... just to get it out of the way.... The mechanics look at us owners as idiots anyway... they wont listen... One trakc minds most of them have.... they can only go down the check list.... a very LONG $$$ check list.



Rob
 
Well, I figure if I'm the one writing the check, we'll do it my way and if I tell them I've changed the filters, then we are not going to pay them $100-$150/hour to do it again......but maybe I'm too simple minded.


Ed,

Yes.........if the Racors trap at more then 10 microns, you are passing some contaminents on to the engine mounted secondary filter. Since they are 2 micron filters, if you pick up junk in the primarys you may well have clogged up secondarys as well.

And, yes, change both the flat gasket and the o-ring on the t handle every time you service the Racors. It is easy to do and they are free and come with every element so there is no reason not to. All it takes is a nick or tear on either one and you can suck air into the fuel system. Purging air from a diesel is a PITA and should be avoided if you can. BTW, air will get into a fuel system thru a hole that diesel will not come out of so you may not see a cut o-ring or gasket before you suck up a slug of air.
 
Ed,
Ur motors will bleed the air in the fuel lines. After changing the filters cycle the start switch or each engine multiple times... DO NOT start just click to ON.... wait till fuel pump stops . then off... wait 20 secs ... then repeat.. u can do both motors together or sep . I do them seperate. When the main fuel filter is empty it takes alot cycles....
I try not to change the main and the racor at the same time as its just that much more air to purge .... ( again i repeat u must here the fuel pump cycle .... if ur too qucik with the cyclings the pump will not come on ) thats why i do one at at time ... how many cycles... over 10 ... maybe even 20... I stopped counting ... sorry ...


Rob
 
UPDATE - So there I was, borrowed strap wrench in hand, filter in the other, new can of diesel sitting next to me, ready to change the primary. Didn't budge. A quick call to Sea Ray and they said that they use a honking strap wrench with webbing, and a breaker bar, to remove/install these filters. Apparently they need a complete turn after being hand tightened? Anyone have a favorite tool for getting these off without losing knuckles? Thanks!

But - the engine ran quite nicely without faltering, so perhaps whatever demons it had have moved elsewhere. Fingers crossed.
 
Ed,
I use a strap wrench as well, but I do not put those suckers on one full turn after handtight- 1/4 to 1/2 turn max, using a 12" adjustable wrench on the square bar that turns the strap.
The first year I did my own annual services I broke a filter wrench trying to get the filters that the yard installed off.
Never again.

Skip
 
I use a metal band type filter wrench. With a strap wrench, the nylon stretches and you cannot feel how tight or what you are doing. I put my filters on as tight as I can by hand then 1/4 turn more with the filter wrench and mine are plenty tight, and I can get them off easily as well.

I'm glad the secondary filters seem to fix your problem.
 
Would something like this work?
This looks a bit odd and I'm not sure how access would be, but the webbing is really thick.

51GYM8FH5XL._SS400_.jpg
 
Ed, that is nearly identical to the one I use-and like Frank I also keep the metal band filter wrench on board as well. The strap wrench works well on the main engine mounted filters, but the fuel filter on the Onan is hard to get to with the strap wrench.

And from my bride: Beat Navy!


Skip
 
Ed, I had to use one of these to get my engine mounted filters off last year. Had no trouble with the generator filter, but my strap wrench wouldn't budge the engine mounted ones.
 

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  • filterWrench.jpg
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Update - thought that the problem was resolved but not so quick - had some fluctuations today on a test run, so I decided to change the secondary. Found that the easiest way was with the seatbelt strapwrench (picture posted above), a 1/2 drive ratchet handle, and an adjustable wrench. Using both wrenches allowed me to keep tension and reposition at the same time. Access was ok with both of these below the filter.

A great suggestion by a boat-neighbor, 1 hr too late unfortunately, was to use a ziplock back to take the used filter. Will try that next time.

Engine runs unloaded fine, we'll see once we get out on the bay. As Capt Ron said, "if it's gonna break, it's gonna break out there!"

Happy 4th everyone.
 
Why don't y'all get a real filter wrench like this one?

http://www.kd-tools.com/35273324.htm

Also, be sure the bag you use is a heavy duty one, not just a sandwich baggie. When the little thin bags fall apart and the mess is unbelievable. I find that placing 1/2 a diaper under the filter and keeping an open top bucket as close as possible does about as good in controlling the mess.
 

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