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      11-21-2012, 02:51 PM   #3
Matt01
Private First Class
United Kingdom
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Drives: F31 320d Lux
Join Date: Jan 2011
Location: Nottingham

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I think it's the particulate filter, it's cleaned out by heat and the engine deliberately running hot... It can increase the mpg by always running... Particularly if you're doing lots of short runs...