I had this same door slamming issue and replaced 2 of the strikers on my W123. The doors shut better but still require extra effort. I am not pleased with how the doors shut on any of our Mercedes. Even my old Mustang's door shuts easier. The doors of our '99 Suburban shut effortlessly.

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85 300D 3 pedal. Current project.
83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.