I think you are reading the manual wrong. Admittedly, I am working off my 380SL's info, but I believe that it is the same system. You should be seeing fluctuation around 50%, and your reading is telling you that your basic mixture is rich. How rich is unclear. IMHE, as you approach 60%, you are nearing the limit of correction. so you may be 'way rich, especially if you have a weak O2 sensor.

You have an impressive list of replaced parts and this was what stood out when I read your post.

You are getting fluctuation, I assume. If it is stuck on 59%, you may be seeing a fault code.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe