Your fuel cap actuator is probably bad, and that could end up burning out your vacuum pump if you do not fix the problem soon. Have the problem checked by a competent MB technician, others will take you for a ride and your problem will remain.

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1939 170 - till I was 3, still turns heads
1989 300TE - daily driver has 263k
1987 300E - purrs happily with 120k
1983 300SD - keeps my friend happy