So I've been pulling my hair out recently trying to figure out why my TI wouldn't idle smoothly and kept giving me a check engine light.
The codes were:
pre-cat 02
fuel trim multiplicative
After replacing my pre-cat 02, and cleaning my MAF the problem still processed. I noticed my water pump was leaking so while I replaced it I decided to completely replace every hose under the hood also. I put in all new gaskets, spark plugs, and wires. The problem was still there! I checked the TPS, idle control valve, coolant temp, replaced the fuel filter, and still nothing!
Finally a senior BMW tech gave me a very good tip. With the car running (attempting to idle) remove the oil cap. If you notice allot of suction when removing it, or a change in the idle then, the PCV valve is stuck open. This creates an elusive vacuum leak. Sure enough that was my problem!
Mysterious vacuum leak can = Bad PCV 
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