Engine Performance problem
2002 BMW 325 6 cyl Two Wheel Drive Automatic 60,000 miles
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where is and how do i change my crankcase vent valve?

It is located under the intake manifold. Frankly speaking, this repair is best left to a trained BMW technician. But if you must I recommend purchasing a Bentley repair manual for your car, the repair procedure may require replacement of some of the connecting hoses as they do become quite brittle as they age.