Howdy everyone. New user/blazer owner. I recently purchased a 01 ZR2 that needed a fuel pump and some other minor things.
I’ve got it running and driving but ever since I first purchased it there has been the lovely hiss of a vacuum behind the dash.
It made the noise on every setting including off and the only time it faded was when I was under acceleration.
I pulled the dash apart yesterday and got the HVAC controls out. I found that the black vacuum “switch” where all the lines meet was were the hiss was originating.
So I’m going to replace that part but I wanted to consult with some others who have more knowledge of the system. As far as I can see there is no fluid in my lines as well. Is this a common problem on these? Or is there a deeper issue causing this to function improperly.

 

So I got around to replacing the vacuum switch and my hiss is no more. The doors don’t seem to fully actuate how they’re supposed to, seems that I still get air out of every vent but it diverts enough that I’m not going to worry until it doesn’t function properly.

 

Good that you fixed the HVAC vacuum leak...however have you replaced the vacuum lines under the hood? Easy and pretty cheap to do. Just purchase a length of bulk hose, reuse all the fittings. Also suggest checking the reservoir that I believe is inside the front left fender. Use a hand vacuum pump at the reservoir slip-on fitting, see if you can pull a vacuum on it and it holds. Also there should be a check valve in the lines somewhere, make sure it is not leaking and working.