I had this problem with both my 300 SEL and SD. On my SD, it was a $5 pin, but on my 300 SEL, I needed something called a fuel governor or something, I forget which., but it was about $220 to replace including labor.

After that the pedal worked fine. This also helped my gas mileage. I went from 18 to 21 on my 300 SEL after it was fixed.

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'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)