I recently had this problem (not self-centering) on my 91 300D. I had all sorts of suggestions, including alignment and steering damper....it was very frustrating In my case, it turned out to be bad ball joints. Though there were neither loose nor squeaking, they were in fact bad. As a safety precaution if nothing else, if your ball joints have never been replaced, replace them.

Anyway, either your alignment is off or something is binding, preventing the steering from self-centering.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 179k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 77k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Thanks I have had the car aligned and have replaced all of the tie rods and ball joints. Unless something was bad from the factory parts ( a possibility) I don't think that it is a bad ball joint. My latest guess is the steering box. I have one on the bench that has a flaw. I am going to study it and see if I can figure out what it is that causes the steering to "bounce back" If anyone knows them intimately I would love to hear about how the work.

Call C&M Hydraulics 888.407.6900. MB steering box experts. Extremely helpful.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 179k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 77k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife