I read the post with the gentleman having a high rpm idle that cannot get it to come down. Machine Man provided detailed information about the IAC valve and setting the base idle. My wife's car is a 1990 240 with 33,000 miles. We purchased it over a year ago. Great car, but I never thought the uneven idle was quite right even though my indy mechanic said all these 4 cylinders idle like this. And, this was after tune-up, throttle body, IAC cleaning, vacuum line replacement where necessary, new O2 sensor. Now this idle was not bad, but you could feel a very frequent blip or perhaps miss. This is subtle. I don't think my wife really notices and this is her car. For comparison, I drive a 12 year old 5 series BMW with in-line six. I feel and hear nothing at idle.
Anyway, I had adjusted the base idle (LH 2.4) per the Bentley manual. Machine Man sets the stop screw differently and explained his rationale. After doing this and then adjusting the throttle position switch, the idle is better. I rate it at a 8 of 10 now compared to 6 earlier.
Thanks Machine Man.