ok so when i turn on my air inside it comes on just fine blows strong but when i switch it to warm or hot air it suddenly makes a really irritating hissing sound, the heat still comes out the vents but it makes a hissing sound almost like air coming out of a small hole exept alot loounder like a pssssssssssssss it stops as soon as i turn the air to cold, car is a 87 z28 anyone know what the hissing noise is it kind of sounds like its coming out were the control unit is and were the radio is

the vents are vacuum operated. Sounds like you have a vac leak .

as well.

You're gonna be pulling your radio console out and part of the dash as well. Make sure you get a good harness to replace it with. This is one of those things you don't want to do twice.

The problem is a 6 or 9 port vac switch behind the ac/heater control assy.

heat console and radio are out now what? what do i look for?

A series of mulit-colored vac lines going into the pass side of the HVAC controls. The switch itself is prone to leaking, so before replacing the lines, check the switch.

JamesC

thanks for all the help everyone but which side is the pass side?

Passenger.

JamesC

wow that made me feel really stupid thanks

Brain fart--we all have them.

JamesC