I noticed today while on a short 30 mi. trip, the ol' cummins was just not purring sweetly as it normally does. I thought it might be the fuel, but the first half of the tank was fine. I don't know how to describe it; a little more injector noise, not quite as responsive to speed changes... . a little sluggish. I have noticed it before and then it runs fine. 45k miles, normal service, running empty. Any clues?

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