Erratic Idle QSB5.9 380hp

[Matthew Swasty]

I just purchased a 2006 Meridian 411 with two QSB5.9 380hp. (400hrs) I had the local Cummins service shop do a complete service on both motors and they said they were in great shape. (They also installed two Seaboard water pumps and were very impressed with them) A few days later, I set out on a 100 mile trip and I ran them up to 1000RPM and I noticed that the starboard motor didn´t come up to 1000. It stayed at idle around 600rpm. As I pushed the throttle higher on the starboard motor it did come up to 1000rpm but the throttles were not in line like normal. After 10 minutes, the throttle dropped back to idle and would jump between idle and 1000rpm about every 30sec or so. Scared me when that happened. It did this for about 30 minutes and then normalized and responded like it should for the rest of the day. The motors both run smooth at the RPM they were at. It was just hard to keep them at the same RPM. I set anchor for a few hours that evening and when I restarted the motors, the SB motor did the same thing and didn´t respond to the throttle like it should. It needed more rotation to get it to 1000RPM. Any ideas? Thanks in advance!

Matt

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[Rob Schepis]

Did you contact the Cummins shop that had just done all of the servicing do advise them of this new issue?

Post some photos of your displays and your controls.

[Matthew Swasty]

Thanks for replying! We did take it out and they hooked each motor to the computer and ran through the RPM’s and did a blowby test. It ran great in the sea trial. A few days later…. It’s like the starboard motor just didn’t know where the throttle was set at and would jump between idle and the set RPM of the throttle levers. After about an hour of this it went back to normal throttle response. My first indication that something was wrong is that I had to rotate the throttle quite a bit before the rpm’s increased up to 1000 to match the other motor. After about 20 minutes in that throttle orientation, SB motor started jumping between idle and 1000. If I went back to idle on both motors, it ran smooth at 600rpm. Is the throttle mechanical or does the computer control throttle response? Thanks again for the help!

[Rob Schepis]

When you set out a few days after the servicing by Cummins was this the first time the boat left the slip after their work? Or did the servicing by them include a final sea trial (aka “bay test”)?

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