A/C coming out of heater vent.

[basherbob]

During acceleration the air flow switches from the dash to the heater vents but comes back when coasting. Checked the check valve. Cannot blow through it but can suck air through it.

Thinking that since it takes a few seconds to switch from dash to heater vents there may be a leak somewhere that allows to system to lose pressure after the vacuum stops. Also hear hissing in the dash during idle but found no loose connections or busted tubes. Can not find source of hissing. Seems to be on passenger side in the dash.

Any suggestions? Would this have anything to do with the vent door creaking shut shortly after shutting off the car? Is the diaphragm prone to leaking and is it easily replaceable? Any routine known problems of this sort?

Sweating it out under acceleration but cool when coasting.
Bob..............

 

[basherbob]

anyone?

 

[1970gsx]

I had the same hissing sound for the longest time. Turns out the 2 hard vacuum lines going from the engine compartment to the control unit had both cracked and had been repaired using small pieces of rubber vacuum line. Trouble was whoever did this swtched the lines causing the hissing sound from the HVAC control unit in the dash. Switched the lines back and all was well . Switch yours and see if this helps the hiss. Mine also swiches to the floor on boost situations even after fixing the hiss. Still checking into that one...

 

[Steve Wood]

Wonder if the vacuum switch on the back of the control head is leaking? I have replaced two of them in my cars in the past year.

Also, the reservoir ball under the left front fender should hold a vacuum supply...wonder if that hose is leaking?

 

[national]

Mine does the same thing other than the hissing..no hissing sound but goes from air vents to floor vents under boost & returns back afterwards..dont think it hurts performance but drives me crazy
Brian