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      08-31-2012, 05:41 AM   #1
RagingKileak
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Small 335i Problem - Off Throttle Squeak

Hi Gents,

Inspired by a new members post I was finally reminded (at a good time) that I am having my own little problem which might be turbo/wastegate related but also might not.

So, this problem began immediately after fitting AR Design catless downpipes. It might have been happening before, but I couldn't hear it if it was.... So it's fair to say that changing the pipes has either caused the problem or (more likely) removed sufficient sound deadening (via the oem cats) for me to hear something that was always there.

Basically, if I rev the engine (car still, moving, at low/high engine speeds none of it matters) then about 2 seconds after I come off the throttle, I will hear an audible 'squeak' from the mid/passenger side of the engine bay.

It's literally like this rev... count to 1... squeak. It's basically like a little mouse - totally consisent, performance not affected (as far as I know) and definitely nothing to do with suspension, wheels etc because it happens even at still. If it is a 'structural' component, then it must be something which moves when the engine revs - like a mount or similar.

Any ideas? I thought it might be a wastegate flap which perhaps needs some grease/oil or similar? That could have been effectively 'wiped' off when the new DP's were fitted as the DTC box made it a painful, time consuming swap which was done largely by feel rather than sight.

Matt
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      08-31-2012, 05:58 AM   #2
nicktyler
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It sounds like it could be something humming at it's resonant frequency as the engine revs drop and vibrate through it if it is that consistent, rather than something rubbing. Does it happen every time at a certain rpm or count to 1 every over-run from any rpm?

If it is at specific rpm you might be able to carefully hold the engine speed and find where it is coming from?
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      08-31-2012, 06:04 AM   #3
doughboy
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Haven't you got forge DVs mate? Mine made a sort of squeaky-plop (?) noise after revving as they close after revs dropped. (they open as you lift off the gas after revving, then drop closed as it reaches idle, then they open quietly after a few seconds at idle again)

Is it a very quick event? or does it go on for say a second?

Don't think its a wastegate issue:

a) they're wedged down by the drivers side inner wing
b) they would stay firmly shut during no load revving
c) they always run totally dry and un oiled anyway as any oil would burn off at 1000degC!
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Last edited by doughboy; 08-31-2012 at 06:09 AM.
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      08-31-2012, 06:35 AM   #4
RagingKileak
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Thanks for the quick and helpful replies chaps.

Nick, I don't think it is anything other than something 'opening and closing' or at the very least 'moving' because it's a consistent 'squeak' (lasts as long as it takes to say 'squeak' out loud) regardless of engine speed, actual speed etc.

DB - the DV's could be to blame here as they are new too and obviously with a lot of recently added 'hollow metal' pipework, it might be that I am just hearing something I couldn't before.

I'll try to listen for a metallic plop (which I think I can imagine having taken apart the DV's and heard the plop you mean when I was rebuilding them with the yellow springs.)

Thanks again.

Rgds
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      08-31-2012, 07:30 AM   #5
Nigelo
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Are there any belts in that area of the car?
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      08-31-2012, 07:57 AM   #6
idnan
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Does it sound like this:



My noise was from my forge DVs. A regrease and rebuild sorted them out.
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      08-31-2012, 12:20 PM   #7
two,_OH_five
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Whip the lid off the airbox and see if it makes a difference to the sound

It could conceivably be a very tiny bit of compressor surge/stall bought about by a change in spool after changing downpipes.

Extreme examples of proper compressor surge

and


for sound comparison but obvoiusly off boost it wouldn't be anything like as pronounced

At low RPM/throttle lift my other car produces a range of amusing sound effects from :-
Did I just fart and not realise
via
There's a flock of pidgeons nesting under the bonnet
to
I think I just ran over Darth Vader


In my case it's mostly down to aftermarket dumpvalve fitment - it's fitted with a much stiffer spring that the OEM one so it doesn't open at low rpm causing very very mild compressor stall
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