BMW 3-Series Exhaust Muffler Flap Valve – Stuck, Noisy – E36, E46, E90, E91, E92, E93
This answer is applicable for many BMWs in addition to the one listed below.
My question to Otto is in reference to the exhaust butterfly valve in the right tailpipe of my 2006 BMW 330I. The valve rattles excessively at idle, so noisy that it can be heard inside the vehicle with the windows closed. I took the car to the dealer who stated that this was a common problem and sprayed the valve assembly down with WD-40 (which of course did absolutely nothing to correct the problem). After returning the vehicle to the dealer, he stated he would have to troubleshoot the problem and would cost $$$$$$. I was thinking that if this is a common problem, there should be a common fix! My question to you is 1). what is the purpose/function of this valve and 2). What can be done to correct the rattling at idle. It does not appear that the valve can be replaced without replacing the entire muffler/tailpipe assembly. The vacuum line to the valve is connected securely. I did not see this problem anywhere in earlier blogs. If I have overlooked it, my apologies. Any help you could provide to help solve this problem would be much appreciated. P.S. I know I could disconnect the vacuum line to fix the problem, but would like to have the valve functional (but quiet) if possible.
BMW has used the vacuum operated exhaust valve on various models. Most notably, many of the 6-cylinder 3-series models from 1996-on. The valve is intended to decrease exhaust sound (noise) at lower engine RPMs, by routing the exhaust flow through a longer (and more baffled) path inside the muffler. At engine speeds above a specified RPM (such as 2500 RPM), the engine management system opens the valve which allows a free-er exhaust flow through the muffler, assuring full engine power.
We have never seen these valves stay functional for the life of the muffler. Typically, they corrode and/or rust and freeze up. Since your valve is rattling, it may not be frozen up, but may have a worn shaft or the vacuum control unit may be faulty (more on this, below).
In testing these units, remove the vacuum line from the valve and test as follows:
* At rest, the valve should be open (engine off, or vacuum line disconnected). If it is not open, either the vacuum actuator is stuck or the valve itself is stuck and/or seized due to rust or corrosion, likely.
If the valve is not stuck closed, to begin with, continue testing:
* At idle, the vacuum line should show vacuum (using a simple vacuum gauge), this would be when the valve should be closed. If no vacuum, there is a problem with the vacuum system and further diagnostics will be required. If there is vacuum at idle, rev the engine past 2500 RPM or a bit higher (not all models are set to open at 2500 RPM, some may be higher …. get the RPM up high, but don’t just bounce it off the rev-limiter). You should see the vacuum go away at some point (this is when the valve would open). If you do have the initial vacuum, then this part of the test should show proper function as well.
* Using a vacuum gun (such as a basic Mity-Vac tool), apply vacuum to the valve control unit. The valve should close. If the valve does not close, either the vacuum actuator is faulty or the valve is stuck open.
In the case of a faulty vacuum actuator or stuck valve (or rattling valve), these parts are not available separately (they are part of the muffler assembly). If the valve is non-functional, it would be preferable to have the valve always open as opposed to always closed. A rusted valve assembly can often be forced open or the flap can be bent to allow the pipe to be open. We really do not have an operational fix for a loose or rattling valve. Perhaps the valve flap could be spot welded in the open position, to prevent the rattling.