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      02-21-2017, 10:58 AM   #43
General Dogsbody
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United Kingdom

Drives: 2017 340-6MT
Join Date: Feb 2014
Location: Bristol

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Originally Posted by smithf80 View Post
It is completely normal for exhaust gas to come out of only one of your two exhaust tips, and for only one to look dirty most of the time. This is because the exhaust on our cars is valved, meaning it has a "flap" inside one of the pipes, and can be controlled electronically by the car for various reasons. In most cases, the flap will be closed, and this is the default setting for both Eco Pro and Comfort mode, this is why you see one tip clean and one tip dirty. In Sport and Sport+, it is open.

Here is a summary of the function of the flap (to my best knowledge):
  • Flap closed on startup to help warm up the cat's faster.
  • Flap will open itself when the car reaches idle, and will typically open itself when the car is idling.
  • After driving away in either Comfort or Eco Pro, the flap will close, and remain closed unless you give the car hard throttle.
  • The flap will remain open in Sport and Sport+ mode.

If you car doesn't do what I described to the T, don't worry, it doesn't mean somethings wrong, the car's computer is very smart, and only under certain conditions will that flap open itself, so if you go out and start your car and it happens to be open, don't worry, maybe your cars already warm, etc, etc.
My car does exactly as you describe and I can't see any reason to do the unplug mod when all you have to do is push 'sport' after start up! I suppose it may sound more meaty on start up with the flap open but, apart from that there seems no reason to me to mess with it, particularly as the world class engineers at BMW designed it this way for a reason!!