While the car is idling, open the hood and listen for any whistling sounds. If you do, that is a vacuum leak. You should also perform a smoke test at a shop, which will send white smoke through the air intake and look for where it comes out to find the vacuum leak. If it passes the smoke test, then at least the vacuum leak can be eliminated and you can move on to check for other causes of the system being too lean. It could be MAF, as suggested, and cleaning it with a CRC electronic cleaner spray or MAF cleaner might work. It could also be a restricted fuel delivery problem- e.g clogged injectors. If I were you- I would clean the MAF first, visually inspect and listen for whistling sounds and pinpoint the area where unmetered air is rushing in, and throw in a bottle of techron in the fuel tank to clean out the injectors. These are the easiest things to try first. If no success, then you can look into replacing the PCV (positive crankshaft valve).
Other things to look at after ruling out the above would be dirty fuel filter and fuel pump- easiest way to check would be to measure fuel pressure. The filter is built in into the tank and not easily replaceable (but not impossible).
At some point, you should also replace the oxygen sensors (they are due at 120K miles per BMW).
Last edited by queensfield; 04-17-2012 at 10:46 PM.