I'm helping a friend with an unstable idle in a 95 E300. Of course I'm doing this by correspondence so it's that much more difficult. As described to me the idle of this E300 is similar to a late model US-spec gasser that's been running leaded fuel for a few thousand miles. Since I have no experience with that, I imagine it's something like what happens when a cat and O2 sensor plug up.

I'm reading good stuff in the archives by Mark Herzig and jdlow but nothing specific to unstable or rough idle. What I've read has to do with problems when running. My friend hasn't noticed any problems under throttle or under load.

Any thoughts on where to begin?

Thanks,
Sixto
95 S420
87 300SDL