Hello folks, new to the forum with a 91 firebird , it only blows hot air from the heater at wide open throttle , why ? i can hear a little hissing in and around the ac switch in the dash so i know i have a switch bad will this also cause it to only blow hot at wot ? or do i have another problem?

 

Welcome!

now, when you're not at WOT, can you feel heat coming from the defrost vents toward the windshield? You may have a vacuum leak (the hiss sound you mentioned) and the car defaults back to defrost when there's no vacuum to reroute air in the heating ducts

 

the air switches from defrost to floor to dash ok but slow , acceleration (low vacuum ) does not seem to cause a problem . it only puts out hot air under hard acceleration

 

i also flushed the heater core (it was clean) im wodering now if there could be a blockage in a line somewhere

 

is the vacuum line to the heater bypass plugged in
in the pic there a heater bypass value and a line goes into the top of it

 

yes it is plugged in . it does have vacuum . and i did replace it earlier thinking that might be the problem

 

well it maybe stuck open or the value inside is bad
and also so see if the ac switch in the dash needs to be replaced maybe its not closing the bypass value

 

ok folks ya'll gave me some pretty good ideas and at least pointed me in a better direction , so thanks . just to let you know i changed the vaccum selector switch in the dash and so far all is good , the vents blow where you want and hot or cold . again thanks