Knocking Noise

[Quitsa]

One of my new QSB 6.7 550 engines makes a very distinct noise audible at idle up to about 1000 rpms that sounds sort of like the old fashioned “Diesel knock.” It is loud enough to be heard above the other engine noises but not much. It goes away immediately if the engine is put in gear.  The other engine does not make the sound, nor did the pair of QSB 6.7s I had on my old boat.

The Cummins Atlantic people have had the diagnostic computer on and everything shows normal.  Cutting out injectors one at a time seems to produce a small change in the sound when #2 is cut but when they capped each injector fuel supply, there was no change. The valves have been checked.

By now the mechanics seem quite totally baffled.  The coupling is a Vulcan and does not seem to be the source based on listening with a stethoscope.

Have you ever run across anything like this sound?  Any ideas for what else to check?  The engine appears to run normally.

Topic

[Quitsa]

Thanks.  I asked the mechanic and apparently one of the things they tried was to unplug the fuel pressure sensor, which puts the pump to default mode of 12,000lbs. He also overrode the actuator raised the pressure to 24,000lbs.  Neither of those changed the noise.  Of course that is not the same as replacing those components but I wonder if what he did would have demonstrated a fault?

[Tony Athens]

I asked my best guy and he said he had what sounded like the exact same issue.. Replaced both the fuel pressure sensor & the fuel pressure actuator–Problem went away..

 

 

Tony

[Quitsa]

Thanks.  I should add that the engines have about 15 hours now and the noise has been present in the one engine from the beginning.  The Cummins tech who did the installation and sea trial checks did not even notice it until he came back after I had pointed it out. It has not gotten any louder or changed at all.  We have done several sea trials testing props that included short WOT periods and I took a small trip that involved a total of about 6 hours running at a 2600 rpm cruise. As I mentioned, they both run fine and temperatures, boost, and fuel rates are very close for both.

[Tony Athens]

I’m checking with my best field tech on this.. I will get back with you..

 

Tony

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