Need help): weird air noise

[Twinturbo2800]

hey guys. Im hoping someone can diagnose my problem. When im driving hard or normal when i let off the throttle i get a air-horn/rubbing sound. I can still hear the BOV so i dont know if it has anything to do with it but i wanna sell my car soon and id like to fix this problem before i do.

simple as a air leak? or somthing more serious? ?


thanks, josh.


BTW. im Automatic.

 

[Twinturbo2800]

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[Twinturbo2800]

no one ??

 

[kenamond]

Test your BOV. Make sure it's working. If it isn't working, you may be hearing compressor surge. That's bad for the turbo. Check the vaccuum hose going between the BOV and IM for leaks. That's where the BOV gets its signal to open. The procedure for testing the actual BOV is in a Haynes manual, but I'm guessing you just remove the BOV from the UICP and recirc tube, put a vaccuum tester on the BOV nipple and apply vaccuum until you see the internal valve open. I don't know what pressure (vaccuum) it should open at, but if you post the results, someone will know if it's good/bad.

If you have an MBC that's T-ed into the BOV line (not the best way to plumb your MBC, by the way), try routing the MBC from the compressor outlet and using a single line from BOV to IM.

 

[Twinturbo2800]

yea its not Tee'd to to the BOV. the sound is different then the BOV air noise... i can hear both when it happens

 

[kenamond]

yea its not Tee'd to to the BOV. the sound is different then the BOV air noise... i can hear both when it happens

The purpose of the BOV is to relieve pressure between a boosting compressor and closed throttle plate (like when you let off the gas to shift). The BOV opens based on the sudden vaccuum in the intake manifold (engine is still drawing air, but the throttle isn't letting any get through). This lets the compressed air recirculate back into the intake pipe for a 2nd go at the compressor.

If the pressure is not suffiiciently relieved, the air stagnates on the outlet side of the compressor (it has nowhere to go, so it stops). The compressor is like a bunch of wings "flying" through the air in a circle. They need the air passing by them to be above a minimum speed or they'll stall just as an aircraft will stall if it's going too slowly.

So you have two things in the compressor: very high pressure and stall. The stalled compressor makes a sound described as a tweet or chirp or fart. I've never heard it. But the high pressure is hard on the seals and bearings of the turbo (the compressor wheel is pushed toward the turbine side by this high pressure adding excessive load to the bearing(s)).

So you may have a bad BOV that is recirculating some of the air, but not enough to prevent surge. So you hear the BOV dump, but you also hear surge. And it only happens when you're boosting and let off the gas.

So check the BOV and vaccuum lines.

That's my guess.

 

[Twinturbo2800]

Ok man, thanks so much for your post, that deffinatley sounds right to me. i think my buddy was messing with the BOV awhile ago and tightened it up too much. possibly why?