Cummins QSC 8.3 runs rough at idle speed in gear, clears above idle.

[Bill Harrison]

My 2014 Back Cove 37 single QSC 8.3-600 HO with 458 hours runs very rough when in gear at idle speed, enough to cause the boat interior to shake.  Advancing the throttle slightly stops the problem. Here are some particulars:

• When started (ZF 286A 0.91 gear not engaged), the engine idles smoothly @600 rpm.  Varying engine speed while in Warm Mode is also smooth with no sign of rough operation.  When the ZF is engaged, the engine speed remains stable at 600 rpm, but a vibration commences which is severe enough to cause the boat cabin to rattle.  The vibration quickly <b>stops</b> as the throttle is advanced, when the engine reaches ~650 rpm or above.  The vibration reappears when engine rpm drops below ~650 rpm.
• In gear operation at engine speeds above ~650 rpm up to WOT appear normal, with no abnormal vibration.  The engine does not smoke, exhibits normal oil pressure throughout the operating range, does not overheat, and by comparison to the performance curves closely matches the predicted fuel flow rate along the entire curve.  Finally, the idle speed vibration is more pronounced when the engine is cold vs at normal operating temperature but under either condition does not stop until rpm reaches or exceeds ~650 rpm.

I appreciate any advice or experience with this condition.

Topic

[Tony Athens]

Vibration?

Linear vibration (when your instruments hanging from the ceiling vibrate ,your boat rails move,  or you “feel it” on the back deck or in the seat of your pants) is totally different from “gear rattle” which can be very annoying sound wise. Gear rattle is typically related to a worn out torsional coupling or mis-match at lower /idle RPM’s and goes typically away when the RPM is raised a tad,.  I guess some boats could  have both?

 

 

[Brad Arrington]

My 2019 BC 37 is similar.  Vibration is perhaps not as severe as yours, but it does vibrate at 600 at idle with the gear engaged.  <br /><br />I nudge the throttle a tiny bit and the vibration disappears.  

[deraldlary]

My Back Cove 37 has done the same thing for years.  I switched to using 15w-40 oil in the transmission and it seems to reduce the chatter (I might be imagining things though).

[Tony Athens]

NO !

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Bill Harrison

[Bill Harrison]

Tony, Thanks for your input. Would raising the engine idle speed set point from 600 to 650 rpm (the point where my QSC 8.3 in gear vibration issue clears) improve the situation?  Would it cause other problems?

[Bill Harrison]

Frank, thanks for your reply. The consensus seems to be that this condition is “normal” for several Cummins engine models.

[Tony Athens]

Vibrations and / or gear noise at low idle speeds

 

What you are experiencing is quite normal in many boats. Some are worse than others and some just happen to be extra nice at low idle speeds. My own boat with a QSB6.7 550 coupled to a remote mounted ZF286IV is not happy as to gear noise (rattle)  much below 750 when in gear — It’s just the “way it is” . Next winter, I may swap torsional coupling types to see what happens..

[Frank]

Sounds like my Tiara

I have a pair of older 6CTA8.3 Diamond Series in my Tiara. They act the exact same way. 

1 user thanked author for this post.

Bill Harrison

[Bill Harrison]

Hello Steve.  The ZF data plate is stamped 2.39;1. Good catch on my typo. I have considered having Cummins raise the idle speed to 650 rpm but want to make sure there is no other underlying issue.  I sea trialed a Saber 38 several years ago with 6BTA-5.9’s – they exhibited the same behavior, so this may not be limited to my QSC 8.3.  Thanks for your input. BH

[Steve Lewis]

BH,

We have twin 480CE’s.  A much older version of your QSC.  On cold start port engine idles at 600 and the strbd engine idles at 630.  I use our throttle sticks to bring them both up to 650 and they behave much better when shifting.  There is an idle adjustment somewhere.  I am lazy and have not gotten around to finding it and adjusting things yet.  So I use the throttle sticks to do it.

What ratio is that ZF?  The Back Cove factory report shows a ZF286A with 2.31 reduction.

[Author]

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