Fluctuating RPM's [Archive] - Bimmerfest - BMW Forums

: Fluctuating RPM's

03-14-2009, 11:19 PM
Hi all, I'm new to this site as well as a new Bimmer owner. I have a 1980 M10/E21 5 speed w/190k miles. Overall, it is in good shape and running order except for an idle anomally. On start up, the RPM's fluctuate from about 700-1100 in idle. On acceleration and running at speed, it is fine, this just happens at idle and happens until it is at full operating temp. It also belches a little bit of blackish smoke at start up and only lasts around 30 seconds then goes away (which normally means a rich fuel/air mixture). I'm thinking that this may be an old O2 Sensor. Please let me know before I spend $70 or so on a new sensor, or if this is totally off the wall and I need to take a new road to fix this as this is my only complaint with this classic.
P.S. I do live in Colorado (above 6500ft), could this have something to do with these issues as well?