OK, here's the deal. I have a 1991 RS 305 TBI. About a week ago it was idling really high. I took off the IAC valve assuming it was the problem. It looked pretty bad. I decided to just buy a new one as opposed to cleaning it because they're not that expensive and this one was about 20 years old. After I replaced it, the idle was right back down where it should be. However, the throttle body is making a loud hissing noise. It sounds like a loud vacuum leak, but its running awesome. The only thing I did was change the IAC valve between the time it wasn't making the noise and when it started. And yes, I tightened it good and replaced the gasket, I'm sure its not leaking around the base of the IAC. What is this noise?

 

I'm still new to the TBI motor, and I have the same issue after the new IAC. Mine only does it occasionally, and only on cold start-ups. Mostly everyone tells me it depends on what kind of intake and filter you have which can make this already existing noise louder, but that doesn't explain why it got so loud after the new IAC. I'm doing a throttle body rebuild today, so I'll see if the sound exists afterward.

 

Is the air cleaner off when you hear this hissing? It might be from the air that the IAC is now letting in to control your idle that wasn't happening before. Did you reset the IAC like in this technical article? https://www.thirdgen.org/tpimod2

 

check/replace vacuum lines . the hissing you hear is most likely a vacuum leak . good luck .

 

where are the vaccum lines?

 

There is a sticker on the underside of the hood when you raise it called something like emission hose diagram that should lay it all out for you

 

I have an after market hood

 

The sticker that would be there looks like this http://i71.photobucket.com/albums/i1...carsvacuum.gif