380SL blues...

[Pili380SL]

Please be patient with this newbie. I tried the search function and though I've learned some, I'm not sure as to the source of my problem. My 1983 380SL (167,000K) had the frequency valve unplugged when I bought it. It runs o.k., hard starting when warm, but recently has taken to stalling on quick stops. Plugging the frequency valve back in causes a rough idle and it won't idle at all until it warms up. It also has a bad hesitation off idle with the valve plugged in. The valve does "buzz" when plugged in, by the way. From what I've read here, I suspect the oxygen sensor or the frequency valve itself? The car blows black (carbon?) at start up, actually marking the floor. Can the O2 sensor be checked at the connector? Thanks in advance for any help.
Paul