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Is your butterfly valve closed? If so, you should notice that the dirtier pipe should be the one that doesn't contain a valve on it. Same situation on my 335, valve side is cleaner.
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335i E93 Le Mans Blue/BMW Performance Shifter
328i E92 Space Gray/BMW Performance Shifter/M-tech Lip/LED Halos
325Ci E46 Titanium Silver
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