I got rid of my crappy 92 RS a couple months ago and got myself a really nice 91 Z28. The car is absolutely wonderful. The one minor annoyance is the hissing noise in the dash. It sounds like it's coming from behind the gauge cluster. The noise comes on after the car starts up and goes out about 5 seconds after it's turned off. Definately sounds like an air leak. My A/C doesn't work but I'm pretty sure it just needs to be recharged. Also, no matter what mode I set on the climate controls, it blows air out of the vents by the windshield. Doesn't really matter because I drive with windows down and get plenty of airflow that way. However, the hissing constantly gnaws at me just because it's there. Anyone what it is and how to get rid of it?

Thanks in advance

 

Dents, welcome to Thirdgen.org

My vote goes to a vacuum leak behind the dash.

The HVAC routing "doors" are controlled by a series of vacuum actuators. Meaning, when you switch from, say, vents to defog, to floor, etc, these little diaphram motors open and close the associated doors. All doors are vacuum operated in this manner except for the main heat door, which is operated by a direct cable. A leak in any one of these hoses could cause the "hiss."

The HVAC switch itself that directs the vacuum to the individual doors is also vacuum operated (draws from manifold vacuum).

For normal operation, you usually hear a slight hiss as vacuum is rerouted when you switch the HVAC controls. If you hear a constant leak, there is usually a leak in one (or more) of these hoses. The valve in the HVAC could also be broke/disconnected/leaking.

If it's just the HVAC control, the lower console plate can be removed to gain access to the HVAC control panel. A junk yard would have a replacement. If all else fails (and this is the cause) i have a "spare" from my Camaro. To access the rest of the hoses, the whole dash assembly must be removed.

*edit* Didn't read clearly enough. You say that no matter what you set it to it only blows out the defog vents? Then look to the vacuum line coming off of the intake. If there is no vacuum signal picked up, the default setting is defog so that a safe driving environment can be maintained.

I'm assuming TPI. If so, the vacuum port for the HVAC is on the right side of the plenum, just behind the port for the fuel pressure regulator. Follow this line to the firewall. Look for a disconnection, crack, melted hole, etc, anywhere along this line. Heck, make sure there's a line there in the first place.

 

Chances are if he's hearing a hissing sound from "behind the dash" its one of the lines behind the AC controls and not in the engine bay. I replaced all of my vacuum lines a week or two ago, mainly because the line just mentioned above was almost torn in half at one point (major leak! c/c didn't work and neither did a/c controls). I didn't hear any noise, and you probably wouldn't from that leak because the hose is so big...so check the lines behind the AC controls.

 

If you don't hear any noise when selecting the a/c and whatnot, the damage on this hose is on the engine bay side of the fire wall.

He hears constant hissing. Either a damaged hose, loose connection, or a failed ported valve in the HCAV cluster. In any of those situations, you are going to have to remove the lower radio/HVAC trim and then unbolt the HCAV controls. I beleive the control valve can be dissasembled and cleaned. It may be as simple as that. However, it could also need replacing, in which case, I'm not sure if a dealer would have it.

 

Well I took out the stereo and then the whole console and checked the HVAC controls. Everything looked OK but the hissing was louder when I took off the plastic trim plate, so I figured I was getting closer. After a couple hours I found the hose that is leaking. It was sticking out under that gas pedal. It's a pink hose that was cut in two and then both sides are held together by a little transparent rubber hose. The problem is, that little transparent hose cracked, due to old age I guess. I tried taping the whole thing, but electric tape doesn't seem to be holding it together well. The whole thing still hisses like crazy. I also tried using a piece of a drinking straw, but again I have to tape it and tape doesn't hold well. The fact that I am upside down under there and its like 100 degrees outside and my hands are sweaty doesnt help either. Any suggestions?

Thanks.

 

Go to the parts store and get some of those little rubber ends that you're supposed to use for that thin plastic line, and make up a new splice.

 

Hehe yea duh why didn't I think of that. Thanks! I'm leaving now ...

 

I was in Melbourne a couple of weeks ago.... who knows, maybe I saw your car.

 

Hehe no you didn't, I am in Fort Lauderdale for the summer

Fixed the leaking issue. The store looked like Publix the day before a hurricane so I came home empty handed. Tried again with tape and it's holding. Now I can choose whether I want the warm air to blow in my face or into the winshield

Thanks a lot to everyone who helped!