My '91 300E idles now as perfect as I would expect it to when it was new. However, it got progressively better as I replaced parts and not just from one or two parts. Biggest improvements came from ICV Hoses which had cracks in them and were hard as plastic (biggest idle improvement), replacing O2 Sensor (stopped the hunting idle), fuel injectors (better acceleration), and EHA Valve (which, I'm not sure was bad in the first place but could have been previously messed with and is very expensive for the size of the part). I also replaced all of the vacuum lines which was very easy. OVP did nothing. Needless to say I had tried new plug wires, coil, and plugs first with little noticeable difference at the time. I have 211K on it now and bought with 170K. Next thing is the cap to help the cold start, two cranker. When I start it now, it immediately idles at about 800-1000 rpm then drops down to about 600-700 rpm when warm.

I also replaced all the hoses and fittings during the rebuild, but not the O2 sensor--though it was replaced a while before I started the rebuild of the engine.. Might be worth the few bucks to give it a shot.

Tnx,

Don

O2 wasn't that hard to do. I bought a similar one ('05 Mustang Bosch) and spliced it in.

I know where it is and how to do it. Will take a shot at it this week. Thanks.