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  #17  
Old 01-26-2013, 05:56 PM
bmwoem1 bmwoem1 is offline
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Location: New York
 
Join Date: Jan 2013
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Mein Auto: X3
The reason why these passages get clogged is due to leaking exhaust valve stem seals. The oil leaks down the back of the valve and eventually becomes carbonized due to the heat. The secondary air holes are right next to the exhaust valve guides. I've done this job many times (I'm a BMW tech)
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