'88 190E 2.3 - low idle, smoky exhaust

In my '88 190E 2.3 with 154K, I have a very low idle (~500RPM) which stalls the engine at times. There is dark (not blue) smoky exhaust (with sooty deposits) but on the highway (higher speeds) everything works well. There is
no oil leakage anywhere (on the ground/around engine) and no blue smoke. A friend mentioned that the 'idle' setting is 'low and rich' and the 'remote sensor' may be at fault. Any comments, caveats, cures, etc... most welcomed. Thanks.
SBC
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