This is worrying me because I don’t know how serious it is…The truck runs fine and gets the same fuel mileage that it always did and there are no codes…It has the Allison transmission and the Duramax 6.6 and it is a 2005 model C4500 Top Kick capacity 15000lbs. The truck has 254780 miles on it. I can minimize the surging by putting Marvel Mystery Oil in the diesel. Some times it is worst than others …Sometimes it goes away altogether…Sometimes it will go away if i shift it out of drive at a stoplight…It happens the worst when I have been at speed for a hundred miles or more or less. Very mysterious…The computer must be sensing a fault and speeding up the idle a hundred rpms …Help
The engine I believe runs if oil pressure is adequate, if you have oil sensor or pressure problems that could be a cause.
Probably a bad fuel pressure regulator.