Just wanted to say thank you for all the info and advice on changing the o2 sensor.
I used the Ford sensor as recommended on various posts... saved a heap of money and replaced it myself. Fairly straight forward job and huge difference in the car now. No more rough/surging idle, no more hesitant acceleration, no more stalling (usually stalled when I shifted from reverse to drive).
Great forum...
Thanks again to all...

 

I am driving a 200E
May I know where is this sensor located?
Thanks

 

To get to the sensor you have to get under the car... It's almost directly underneath the gearshift/console... It screws into the exhaust system (on top) just before the catalytic converter. There should be a thin curved metal shield that clips onto the sensor to protect it. It has three wires poking out of the top, these go into the car through a rubber seal in the floor. You have to wait until the car cools down before you can touch it or the exhuast system. There are three wires that run from the sensor through a hole in the floor/tailshaft arch (theres a rubber seal that stops water coming into the car) and the wires unplug under the floormat/floorpad just in front of the right hand side front seat. Here in Australia we call that the 'drivers side'. In USA they call it the 'passengers side'. I used the Ford sensor which is much cheaper than the genuine benz sensor, but you have to cut and splice the Ford wires to connect the sensor to the original plug ends off the original sensor that you remove... no big deal, I just used three plug joiners and some heat shrink that I purchased for $1.20 at any local electronics store. (radio shack etc). There are quite a few other posts out there on this topic, and they are all worth reading. I now have found that since putting this new sensor in, the car is running much better... still not perfect but way better than it was... I am now investigating the OVP (over voltage protection relay) I see lots of other posts on this.
This item too can cause idle problems. My car 1989 300E 130K... I guess its inevitable that parts are going to wear out and break down... helps if you can DIY...I found this place to be really helpful and prices are great too www.autopartswarehouse.com Also check out other posts... This rough idle problem must be driving people crazy
hope this helps
regards