89 s10 Vacuum Leak behind Dash??

I have an 89 s10 4.3 4x4 which has been running great. The other day when I went to start it up I heard what sounds like a vacuum leak or hissing air sound. Ive checked under the hood several times and can't
find where it could be comming from. It really seems like it comes from behind/inside the dash. When I turn off the truck it (the hissing sound) slowly dies down and stops prob after about 10-15 sec after the truck has been off. The only thing that has made a difference is when I move the switch for the heater/ac vents. ie. from floor to vent, to defrost. I have found a place between the two where the sound seems to be the quietest. When turining on the heat it works fine but there are two distinct sounds one from the heater, and still the hissing sound. Truck runs good and haven't noticed anything wrong with it performance wise. I just dont like having strange sounds. Any help?
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My 92 does the same thing. Never taken the time to rip it apart to see what's up.
LS

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LS wrote:

Vacuum control valve seal block is leaking. It is behind the heater control selector (vent/AC/heat/Defrost) They age and the rubber gets hard and doesn't seal as well. There are a couple ways to repair them. One is to go to the dealer and buy a new control head. The other is to pull it apart and resurface the seal block and tighten it up.
--
Steve W.
Near Cooperstown, New York
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Aren't the ducts that direct the air to either the floor, dash, or defrost controlled by vacuum? My '86 Blazer uses a vacuum switch to select between them, as do most GM vehicles from that era, I'd imagine. Could be a busted vacuum line. If one of the lines is loose or cracked, it would display the symptoms you're describing.
As I recall, those lines are harder plastic, not like the soft rubber lines found underneath the hood going to and from the intake setup.
By the way, there still will be some residual vacuum pressure in those lines for a few seconds after the truck is turned off. On my Blazer, if I move the switch that selects between floor/dash/defrost right after the truck is turned off, you can hear some hissing for a couple of seconds as well. Even when it's running, if I change settings, I'll hear the hissing as the ducts open and close. That's normal...but a constant hissing indicates a leak for sure.
Unfortunately, the only solution is to do a little exploratory surgery to find out where the problem is.
Good luck...
~jp
jiimi wrote:

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Just like ls I've been trying to track down the same thing on mine. If ac is on vents and you accelerate (not heavy) it goes to the defrost vents. When you take your foot off the gas it switches back. Just got done replacing both fenders and then the rear view mirror comes off. sheesh.
md 94 s10 blazer
Jon R. Pickens wrote:

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Odd... Well, the engine IS going to be the source of vacuum in any case.
To me it sounds like your selector switch is bad--like it's got an internal leak. If vacuum exceeds a certain level, it could be leaking into another area of the switch and activating your defrost vents.
Should be a fairly cheap fix.
~jp
vwtopdown wrote:

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The selector switch just cracked and feel apart, Bought my 89 S-10 new, 75k on it now, I took care of it, but they are rust buckets, plastic switches, plastic ages crackes and falls apart, I think GM intented us to keep buying new ones, yaa right...
KenL

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Eighteen years takes its toll...regardless of the mileage. If you did indeed take good care of it, it certainly wouldn't be a "rust bucket" because you would have spent a few hundred dollars ever two years keeping up with the pin hole rust blemishes in the finish.
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They are known for rusting but if you take care of them they won't for a long time. Mine is a 92 and I have 342,000 miles on it. It didn't show a spec of rust until this spring, but once it started it rusted like crazy. I'd say 14 years before rusting is pretty good.
I had the same problem with my vacuum lines but a $3 part fixed it so I'm not too upset. I still have a leak or a bad vacuum actuator or whatever it's called. It will not switch to heat. I noticed there is a little metal arm that hangs down on the outside of the heater vents in front of the 4x4 shifter. It looks like there are two holes in the plastic where there should be screws or something. If I move the arm I can force air through the heater so I'm not sure if that is the cause.
LS

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