Finally gave up trying to solve my 89 300E rough idle (replaced cap, rotor, wires, plugs, OVP, cleaned idle ctrl valve, etc.), and took it to the shop.

Mechanic indicated lamda tower had a vacuum leak at base and was replaced, O2 sensor was replaced, and injector #3 was cleaned and re-installed. Idles much better but still not smooth as glass. Car has 120K so maybe I'm expecting too much.

Here's my questions:

Where is the O2 sensor located (would like to see a shiny new one in there with my own two eyes...)? What is a lamda tower and where is it (thought lamda and o2 sensor were one and the same)?

Thanks for your help.