About a month ago I removed the heater blower motor box because I broke it while installing headers. Well today I got a box out of a junk yard and put it in but now my heater controls aren't working. They stay in the default mode (defrost vents). When you select "hot" you can hear vacuum hissing out from behind the control but when "cold" is selected the hiss goes away. I have the thin/hard vacuum line that comes out of the passenger side firewall hooked up to the intake and when you pinch it off the hissing goes away. I took the controls out of the dash and played around with the back of them to see if I could find a leak, it seems to be coming from the bottom of the controls. Is there some kind of diaphram or something that could have broke? I read through the archives and found similar problems but didn't come up with much answers.

 

You most likely broke a vac line behind the motor. On the passenger side of the motor, along the back of the firewall, near the tranny, you'll see two black vac lines. One of those vac lines goes a valve on the heater core feed line, and the other vac line goes to the back of the manifold. Trace both lines (especialy the one to the manifold) and check for cracks or see if it's disconnected.

 

I have 2 lines, one used to go to the fuel pressure regulator (or maybe it went to the EGR, I forget) when I had TPI and the other was for the heat. The one for the heat is hooked up the manifold and the other one is capped off. No leaks anywhere...

 

back from the dead

i have the same problem, the hiss is coming from directly behind the controls. both my vac lines are hooked up to my carb. when i give it gas the hissing stops due to the loss of vacuum but starts up again as soon as i let go of the gas. what's broken? i have a spare hvac controller at home that i'll try, maybe that'll fix it

 

Hah ...well summer is nearing an end and I'm going to be needing my heat pretty soon. I *still* have this problem. I was going to go to a junkyard and take out all the heater controls and just redo the entire thing ...who knows.

 

i've worked on a couple and the problem on both was the selector dial mess of vacuum lines. i ran a few pounds of air in the feed line from the engine and soaped it with windex, pretty easy to find the leak that way.