Check the locking pin in the gas door cavity. Mine was slightly bent once causeing the door to remain locked. I straightened and lubricated the pin and have not had a problem since.

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E