E92 - Vacuum Control Exhaust Flap question

[T1berious]

Hi,

We've had our E92 330 for a few years and recently it's developed a metallic sounding rasp \ tinging, happens at low revs (<2K rpm) and can only be heard when the windows are down. It dissapears after 2k.

Anyway it has been getting load enough to warrant a trip to BMW, who couldn't hear a thing, I went and drove the tech around teh block and sure enough it started at low revs.

I was told "it's Vacuum Control Exhaust Flap and is entirely normal as when the car heats up the gases build up and open \ close the flap". I was then told it's an internal part of the back box (£300), some people have replaced it but the noise is still there.

I just wondered if anyone else has had this? To there credit BMW didn't rip me blind and charged half hour.

Just curious as trawling the web has brought little joy apart from a golf tee mod

Cheers for any info T1b

[griffter]

i dont quite understand the garages explanation. Are they saying the noise is a function of normal operation or that it is sticking open or closed? In the e36 the solenoid that controls the valve is by the spare wheel well (follow the vacuum tube from the valve into the boot floor). If you disconnect the plug to this the valve should stay open.

[Carmo99]

Just for info: I've got the same car and have never heard this noise. I also drive with the windows down and I am totally anal when it comes to rattles etc.

[JimexPL]

Just pull the vacuum hose off of the back box (a small rubber pipe going to somewhere above one of the tailpipes)and take it for a drive to see if the rattle has gone.
If it has, and you're happy with the extra low rev noise, then find the motor that creates the vacuum (usually a small black box in the boot somewhere) and disconnect it.

Alternatively, every BMW that I have ever owned has developed a rattle from one of the exhaust heat-shields coming loose. These normally wear through at the mounting bolts, and can normally be cured with larger washers that arguably should have been fitted from new.

I don't know if either applies to the E92, but my E36, E46, E39 and Z4 (heat-shield only) have suffered similarily.

[4rephill]

JimexPL said:

Alternatively, every BMW that I have ever owned has developed a rattle from one of the exhaust heat-shields coming loose. These normally wear through at the mounting bolts, and can normally be cured with larger washers that arguably should have been fitted from new.

I don't know if either applies to the E92, but my E36, E46, E39 and Z4 (heat-shield only) have suffered similarily.

+1 On this, I've had to do the large washer fix on the heat-shield on My E36 328i.

Also, I know that on the E36 and e46, the valve is designed to remain shut at engine speeds up to 2500rpm so that the exhaust note is quieter. (See this link: http://www.e36-tech.com/forum/diy-328i-323i-golf-t... )

So it makes little sense for there to be more noise at low engine speeds and quieter at higher engine speeds.

Sounds to Me like something has come loose inside the back box on the low engine speed/longer pipe side and is resonating when the exhaust gases travel through that pipe.

Then, when the valve opens and the gases go straight out the higher engine speed/short pipe side, the resonating is reduced or stops so you don't hear the noise any more.

Just a thought.