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What is your max rpm?
When it drops rpm does it stay there or does it come back, then does it drop again?
What is the serial number of the engine?
So the rpm's are bouncing not staying a a lower rpm? Is this a new symptom or something that has been happening?
Let me check service bulletins and look a few things up. I'll post back as soon as I can.
See if you can run with the scan tool on the engine.
Do you have any alarms or a Smartcheck gauge?
The power comes from the batteries all the time, same as in a car. But the alternator is to provide enough charging voltage to keep everything running