I have a 98 ML and the fuel gauge never showed completely full. Moreover, the tank was empty when the pointer first touched the lower white bar in the gauge. This situation continued without any problem from the fuel pump. Do not fix it just for this minor irritation. While under warranty, we had the dealer try to fix this problem, but it was never fully corrected.

However, I then had to replace the fuel filter as I was approaching 130K. I read all of the threads on the task and did it myself. In the process, I unplugged the fuel pump and noticed blue corrosion on the 12+ Volt contact. When I reattached the plug, the pump was not working. Ultimately, the fuel pump failed and I replaced the sender/pump and the wiring harness. The new pump has an arrow molded into the white plastic, and this marker should line up with an indentation in the fuel tank. The mark should be pointing across toward the right side of the truck. For minor adjustments, when looking down at the fuel pump and after loosening the big plastic ring which secures the pump by one full turn, twist the pump counterclockwise a few degrees to raise the gauge closer to the full mark on the gauge.

Sorry to be so long-winded.