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Hello and welcome to JustAnswer BMW,
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This would most likely be caused by an unmetered intake air leak of some kind.
Unmetered air leaks allow excess air into the engine and cause the fuel/air mixture to go out of alignment.
The definitive test for intake leaks is a smoke test where a smoke machine (kind of similar to a fog machine you would see at a concert or on Halloween) pumps the intake full of smoke and then the smoke reveals the leaks as it seeps out through any cracks or holes.
However, you can still check for leaks at home. To check for intake leaks, visually inspect the intake area for any cracked hoses/pipes. If you can't see anything, you can try this trick: spray a can of carb cleaner around the intake with the engine idling. If the idle changes, this indicates that the cleaner is entering the engine and you have localized the area of the leak. You can then inspect further.
The most common source of air leaks is from the rubber intake tubes between the air box, and the throttle.
Another common issue that causes this problem is a failed crankcase vent valve (aka PCV). This valve regulates a vacuum on the crankcase, when they fail, a high vacuum is put on the crankcase - giving an air leak. You can check for this by removing the oil filler cap with the engine running, if there is a high vacuum holding the cap down, the engine stalls, or idle dips a lot, then this valve has failed and is your problem.
Aside from the above, a failed mass airflow sensor can also cause this, a fuel pump / pressure problem, and also possibly an o2 sensor or temp sensor. However, these would be much less likely than the above items and would also require more sophisticated diagnostics that are provided by a p code reader to trouble shoot.
I hope that is helpful, please let me know if you have further questions, or if I can clarify anything for you.
It has been my pleasure to help today and I wish you the best of luck with the vehicle.
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