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Exhaust flap fix
I am a new Z4 owner with a 2011 35is that has over 70K on it. The first thing I found on the cold start was this terrible rattle from the right rear. This forced me to design a better muffler flap solution that fixes the rattle problem completely without buying a brand new muffler.
The issue in this design is that you have a heat difference that could cause the flap to become tight once the exhaust starts to flow. The designers probably made the clearances on the flap shaft a bit larger because of this. So the rattler is not from the vacuum actuator as some have noted but the flap shaft and the muffler. The vibration in the muffler system is rattling the flap when the right frequency is present. If you simply apply a slight amount of pressure to the flap shaft it will stop the rattle at any frequency.
Using a ~2" length light coil spring with closed rounded ends can do the trick (You can find these at harbor freight, See photo). Bend the ends to form a round that is slightly larger than the flap shaft diameter on both sides. Attach the spring from one side of shaft to the other by stretching it and you now have tension on the shaft. Takes only 15 minute to make and install. Add a small amount of high temp thick grease to allow the flap to rotate freely and you are done...
Hope this helps....
Thanks z4s4
PS: Anyone know where you can get a service manual for this car?
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