As described in post 26, I had a problem with steering not centering. It was caused by the idler arm bearing.

Did you replace the idler arm bearing kit? If so, was it replaced with at OE Mercedes kit or an aftermarket kit, like Lemfoerder or Uro or ?? I had put in a new Lemfoerder in my W123 and it was the cause of the problem. I replaced it with an OE kit and problem was solved. I could explain why. But first check what you have in there.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5