Hi all,my 85 Z28's a/c control makes a hissing sound on heater and defrost.I replaced the whole control head about 9 months ago,because the temp control was sticking.And it worked fine until a couple weeks ago,could the new one have broke already?How would I test this out? Thanks...

 

The hissing is actually a vacuum leak. All the controls work off of engine vacuum. You probably have a cracked hose going to one of the controls on the HVAC panel.

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FORMULA 350 TK
Southern California 3rd Generation F-Body Organization©

 

my car does the same thing whats even worse it i have an ducting leak somewhere and when i have my a/c on cold air blows through my defrost and fogs my windshield it sucks

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91 Z-28 L-98 Black stock 14.20 @ 97

 

I've had the same problem and found two possible (and closely related) causes.

1) The rubbery "manifold" into which all of the vacuum lines terminate can be leaking at one or more of the nipples on the vacuum switch. Carefully remove the round retainers (one on each side) from the plastic studs (don't break one, or you'll have to replace the switch) and remove the "manifold". Clean the nipples and the insides of the manifold. Before reinstalling, apply a film of silicone grease on each nipple to encourage sealing.

2) The round vacuum switch itself can be leaking. There's not much you can do but replace it (although you can disassemble it). It's not very expensive and is available from GM. If 1) doesn't fix it, 2) should.

It worked for me

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'87 IROC 5.7l TPI - original owner!

 

Well let me throw in my $.02... I just "fixed" this problem on mine ('91 Z). I had a spare switch available and it wasn't that (in my case, anyways) I disconnected the tube harness from the switch, checked vacuum with a gauge, and all was good - no leaks. I had 2 options at that point (1) rip the dash apart to replace the entire harness, or (2) separate all the individual tubes and attach each one individually. I opt'ed for #2. Apparently the "pad" of the harness wasn't sealing worth a damn, and after many attempts to get it to, it was much easier to separate the tubes. IF you do this, obviously make a diagram so you know which tube goes where, and then cut the harness "pad" up, separating the tubes. Take a razor blade and trim off all the excess pieces so you have a uniformly looking tube and just attach them puppies one by one. I used the tiny zippy-ties on each one for a little extra protection, but they maintained a good tight fit by themselves anyways. I hope you can fix it, cuz ripping the dash apart wouldn't be alot of fun, I don't think.

KAM

 

I'll have to try these things.The last time I had it apart,you could hear the vac. leak sound change or go away ,when you moved the junction or pad or manifold around.I did check all 4 or 5 hoses with a vac guage but they seemed to hold vacuum.And the factory shop manual doesn't give a way to test this.
I'll let you know what happens...Thanks,

Now if I could only solve my outer tire edge wear in 3000 miles problem,I'll LOVE the car

 

That is the EXACT symptoms I had. Thought I had it fixed a couple of times, and had to go back in. If I'm not mistaken, the purple line is the vacuum supply. When I did mine, I just put a gauge on that line to make sure I didn't have a leak elsewhere - up in the firewall, etc. I held a steady 21" of vacuum. I think what happens is that the rubbery manifold, or "pad" gets all dried up and doesn't want to hold anymore. But like I said earlier, it was alot easier to separate the tubes with a knife than chase 'em thru-out the dash. Good luck with it - just wanted to share my thoughts and experience with you and try to save you some pain. My method of fixing it has already proven itself - no problems.

KAM