I'm not sure of M-B's fuel delivery strategy but I wouldn't expect rack (IP) position to be much related to fuel delivery. Generally speaking on a mechanical Diesel injection system fuel delivery is determined by rack position and load. So even if you held the accelerator pedal dead steady the IP would add fuel if you hit a hill and pull fuel out on a downhill (or any other change in load).

Cat traditionally used a speed priority system where the IP would maintain engine speed regardless of load while other US engine builders (Cummins, Detroit Diesel, etc.) used a load priority system where engine speed would vary depending on what it took to maintain the torque output which was determined by the operator's pedal position. Two different ways of getting to the same point. Again, I'm not sure of M-B's methods.

Hope that confuses the issue!

Dan