one when you hear it. The aero and other turbo cars with bypass system has a sharp and clearly defined momentary "Blap" vice a "hiss" when it unloads after boosting but one must have good hearing and low background noise and windows open. Those cars which relieve to the atmosphere just have a "woosh". The bypass systems if not relieving well have a longer duration higher pitched hiss like leaking air. The Mechanics of the system are simple- the turbo must be unloaded at the output side when the throttle body is closed. if it is slow to do so the pressure remains in the up pipe to the throttle body and at engine Delta Pacross the throttle body attemps to unload through the IAC valve. this will also make strange noises and can lead to early failure of this valve by being driven backwards. In some cases repeated back pressure can fatigue the plastic nozzle at the valve but this will manifest it self with a sharp hgiss when boosting and at release of the throttle. Not many do the what iffing and look at this problem associated with the bypass valve. those systems dumping to the atmosphere do not have this problem. All this is in the ear of the beholder. I had one defective bypass on a friends car tht sounded like a mouse squeek. The valve seemed good upon inspection and after replacing the small vacuum line it worked fine the line had a small restriction in it and was slowing the response time sufficiently to slowly open the bypass rather than rapidly unloading as it should. hope this help a little

• hissing noise after acceleration, chris, Fri, 8 Jun 2001 09:15:33
  • Sounds like turbo bypass valve to me., Aaron Gilbert, Fri, 8 Jun 2001 12:09:55
  • Re: hissing noise after acceleration, TS, Fri, 8 Jun 2001 09:41:36
    • Re: hissing noise after acceleration, chris, Fri, 8 Jun 2001 10:19:30
      • Don't worry - be happy (n/m), Bill Davies, Fri, 8 Jun 2001 17:05:11
  • Re: hissing noise after acceleration, BillJ, Fri, 8 Jun 2001 09:32:59
    • Re: Bill pretty well nailed this BUT if the valve is not, aero guy, Fri, 8 Jun 2001 10:42:16
      • Correct vs. Incorrect Hiss?, 94 9000 CSE T, Fri, 8 Jun 2001 13:29:52
        • Re: you have got to hear the good vice the bad his to know, aero guy, Fri, 8 Jun 2001 16:25:32 <-- Viewing This Message
        • Re: Correct vs. Incorrect Hiss?, TS, Fri, 8 Jun 2001 15:43:39
        • Not sure if the 2.3l differs, but my car doesn't hiss at all.., Aaron Gilbert, Fri, 8 Jun 2001 15:34:56

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