So, I replaced both of my cats (with 2.5" catco cats) today, and I still have a high-load hiss sound that I guess simply must be the sound of the TPI intake, or perhaps the air filter housing or something in that area.
Anyone have any ideas on what's responsible for the hiss, and how to get rid of it? it sounds like crap. The hiss sounds the same regardless of RPM, and I only hear it under load - that is to say, the more i put my foot in it, the more hiss i hear (full throttle at 3000rpm hiss sounds identical to full throttle hiss at 5000rpm). At about half throttle or less, no hiss.

edit: the hiss does not change with engine temp/runtime, and manifold bolts have been checked as tight.

 

Uhhhhhh let me think...Is your car like mine and have the tube from the air pump that runs to the cats? Maybe it's not connected good. I don't really have a clue right now I am trying to think what could be making the noise. Have you been able to narrow it down any farther as to where it's at? Does the car have any running problems? If you can narrow it down a little more as to what area it's coming from, I can probably help more.

 

I just re-read your post, maybe you should check your intake tubing. Sounds like there might be a hole or a leak in it somewhere. Start the car and carefully run your hand all around the intake tract and maybe around the plenum/runners where you can. If it's leaking enough to hiss you should be able to feel a little airflow by your hand. You could also get a can of throttle body/carb cleaner and spray around your intake system. If you spray a certain area and the engine speeds up a little you have found your leak. Sorry it seems I'm shooting from the hip here, but that's a very odd problem. Let me know if there's anything else I can do.

 

vacuum leak?

 

well, i found that my air filter housing is cracked a bit, maybe that's it? I don't know.

I can't duplicate the hiss with the hood open, it seems to need a load to make the hissing sound. on an open highway you don't really notice the hiss, it's only when there is stuff around for sound to echo off of, like barries or sound walls or whatever. So i have no idea where the sound is coming from.

guess i'll do the diagnostic checks of spraying around things. So far I know the car holds a steady 16" vacuum at idle, and doesn't seem to have any problems, well it runs only 93mph trap speed in the 1/4 (as I found out tonight) but i guess that is just the low end of average for an l98.

 

if the car is bone stock with 6* of timing, you should be pulling closer to 20 at idle and around 18 in gear

 

Dont get me started. TPIs have been notorious for vacuum leaks up the yang. I mean, you have TB gasket, 8 plenum to runner ports, 8 runner to baseplate ports, about 6 vacuum lines and you STILL cant find the prob. More of a reason i ordered my StealthRam the other day.

 

This sounds strange but check your timing. If you leave the TPI open, no filter box attached, and retard the timing, the TPI will begin to make a very loud sucking sound.
If not, a can of starter fluid works to find vacuum leaks.

 

My car, and all TPI camaros with cut air boxes really make a very pronounced hissing noise at idle. However, i cannot hear this loud hiss inside the car, much less when driving.

He is also not hearing a vacuum leak, a vacuum leak that bad would have moved his idle speed way up. I mean, a small line won't make any noise when unplugged, and also will not make any noise under load when vacuum is low (as proportionally less air try's to get through the little hose and more comes through the TB. To get an audible hiss, something big like the PCV or brakebooster would have to be unplugged, and he'd be idfling at 1500+ (or at least show an IAC code) if that was the case.

I believe our friend has an exhaust leak, plain and simple. An exhaust leak will hiss under load and be practically silent when not under load. If you want to find your exhaust leak, have a buddy cover your tailpipe(s) with old rags/towels to increase backpressure while you idle the car and the sound should be amplified, as well as the volume (so that you can feel where the leak is)

I bet a nickel we're talking about an exhaust leak, his description just doesn't fit my experience with TPI intake hiss.

 

Ed has a good point. Try that, I think I was way off base. At least I tried . Anywho good luck on finding the leak. If you need any other advice, post again.

Jason