If the O2 sensor has not been replaced, appears to be original or in for a very long time, that is a good candidate for replacement too. On the 126's it is a real piece of cake, maybe 1/2 hour job.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.