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BMW 3-Series Exhaust Muffler Flap Valve – Stuck, Noisy – E36, E46, E90, E91, E92, E93

January 31, 2011

This answer is applicable for many BMWs in addition to the one listed below.

2006 330i
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.

Bavarian Autosport

  1. Robert Tock permalink

    As the originator of this blog, I performed the vacuum tests as you suggested and determined the valve itself was the culprit. Replacing the muffler is a high dollar solution. The easiest fix was to disconnect and plug the vacuum line to the valve which allows the butterfly to remain (springloaded)open. The exhaust note is still quite pleasant at lower engine RPM’s without the rattle.

    • Certainly a viable alternative …. when the muffler is just fine, otherwise.

  2. Istvan Bognar permalink

    Hello, i have a 1996 328i a. And I have the following problem. My exhaust flap valve does not open when you its suppose to.
    Engine off, the valve is open. Turn the engine on and it will close normally. But if you rev the engine up it stays closed. I’ve just bought the car and noticed that there was an alarming noise of a high pitch from the tail pipe in high revs probably due to de valve being closed in high revs.
    So to use the car wile I try to find the solution I’ve left it open by removing the electrical plug of the solenoid on the trunk of the car.
    Can you help?

    • It would seem that the vacuum solenoid valve is staying open and applying vacuum at all times. The solenoid valve should have voltage applied and be allowing vacuum to the flap actuator at idle and low engine speeds (when the muffler valve should be closed) and then should be unpowered when the flap should be opened (closing off the vacuum and opening the flap).

      The solenoid valve receives 12-volts directly from the ECM (Engine Control Module) relay and then the power goes to the ECM, where it is either left open, (no power flow through the valve, valve is closed and the flap is open) or grounded (power flows through the valve to ground, the valve opens and the flap is closed).

      If unplugging the power from the solenoid valve allows the flap to open, then we would think that the solenoid valve is working properly and that the power to the valve is at fault (always supplying power). If this is the case, there may be a grounding short in the wire from the solenoid to the ECM, or the fault is in the ECM.

      You can’t do any harm by leaving the solenoid unplugged, and the valve open.

  3. 330i permalink

    Otto, your a ledgend. After many years of hearing this rattle I’ve just unpluged the vacuum line and noise has stop. Cabin noise is still the same. Love the e46 330i.

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