My '98 E-430 rarely fills up to the full mark on the gauge. It seems to work
better with some gas pumps than others. Often it will show almost full
when I fill it but then drops to 7/8 after driving 2 or 3 miles. I assumed it
was some sort of over zealous "anti top off" feature to prevent spillage.
If your dealer was able to fix the problem for good, please let us know.