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i have a 2001 525i and it runs good but occasionally i get lean codes for bank 1 and 2. i have looked high and low and cannot find where a leak may be coming from. the intake boot is ok. the hoses are all plugged in and not cracked. i did recently take apart the oil separator system to replace something, i never replaced any o rings, my question is could an o ring cause a leak? i can hear it when i have the engine on and im standing directly on top of engine. i thought it was the disa gasket so i ordered a new one and it still does it. when i turn the engine off, for maybe a good 3-4 seconds i hear air either escaping or entering from somewhere, is this normal with the bmw engines? i will rip the entire system apart and change every gasket if i have to, or if anyone has some advise so i dont have to spend my life savings on this project. thanks in advanced
 

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- Poolman's canonical thread on battling vacuum leaks on the BMW I6 with lean-condition misfire DTCs (1) & where to find all the vacuum hoses, tubes, pipes, and endcaps in the E39 engine bay (1) & where are all the fuel injection o-rings (1) & how to locate all the o-rings (1) and gaskets (1) & where in the USA to get new vacuum tubing & end caps (1) & what SAE sizes to get for all the metric M54 engine vacuum tubes, hoses, pipes, and caps (1) & how do we correct the F-connector errors in the realoem diagrams (1) & where do some of those hard-to-locate vacuum hoses end (1) & what is the sorely needed clarification on how the M54 CCV vacuum port works on the M52 CCV valve connection to the fuel pressure regulator connection (1) & how to make, borrow, or buy lean-condition misfire test tools to test for vacuum leaks & lean conditions (1)
 

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The sound of air leaking out after shut off is normal. You also will hear a faint air leak when standing over the drivers side of the engine when it is running,normal again. I would suggest that you look to the back of the intake manifold for the vacuum hook ups that are near the firewall. They are hard to get at, you will need a mirror and a flashlight and need to remove the air duct to see what all is what. If you haven't been back there before,,it's time to replace those hoses and the caps as well. Good Luck
 

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I had the same problem. Couldn't find a leak. Finally changed the mass air flow meter on the intake and the problem disappeared. Can't say if this is your problem, but it is a possibility. However, my local BMW guy said this problem also is frequently caused by a leaky PCV valve, when the diaphragm in it gets old and brittle.
 
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