The gas pedal has quite a resistance and makes the right leg tired on long drive because I have to permanently maintain an amount of pressure on it. It is normal with this car or something needs fixing?

Thanks!

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1996 E320 since 1/16/08, 171K miles as of Feb 2011

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1989 300CE - R.I.P. Dec 29 2007
Other MBs (sold): 1992 300E-24 - 1979 350SLC - 1984 230E - 1990 300CE