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      01-30-2016, 04:02 PM   #1
smithf80
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Drives: '18 F80 M3 6MT
Join Date: Mar 2014
Location: Long Island, NY

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2018 F80 M3  [5.00]
Before you ask why your exhaust is broken and why only one side is dirty: READ THIS!

This question comes up all of the time, so I have asked a moderator, Dackelone, to sticky this so it can be at the top of the forum for everyone to see, although I'm sure this won't stop people from asking still.

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.

Hopefully this thread will help put to rest all of the questions about this function so we can all get back to complaining about other small, unimportant things.

Last edited by smithf80; 03-12-2016 at 09:33 AM..