Heat/ac control unit

[94HeavyChevy]

when i turn my heat or ac or vent on in my 95 caprice it gives a slight hiss like its releasing pressure or something. Recently the hiss doesnt stop for about 3 minutes or so and i can only get air to blow out of the vent setting. I i put it on heat which is to the floor or front defrost it still comes out of the vent for a few minutes then finally switches over. Do i need to replace the unit?

[j cAT]

Originally Posted by 94HeavyChevy

when i turn my heat or ac or vent on in my 95 caprice it gives a slight hiss like its releasing pressure or something. Recently the hiss doesnt stop for about 3 minutes or so and i can only get air to blow out of the vent setting. I i put it on heat which is to the floor or front defrost it still comes out of the vent for a few minutes then finally switches over. Do i need to replace the unit?

when these vehicles are very cold this is normal...I do not change the heat controls normally when the vehicle is very cold like now....

after 5 min of operating and the air box starts to warm then change the settings...

[94HeavyChevy]

it happens when the car is warm as well. Wont switch to defrost or heat for a little while. Everything blows out of the vent. So if i start my car to let it warm up is it bad to have the heat on?

[j cAT]

Originally Posted by 94HeavyChevy

it happens when the car is warm as well. Wont switch to defrost or heat for a little while. Everything blows out of the vent. So if i start my car to let it warm up is it bad to have the heat on?

with the blower at the lowest speed change air venting select.
the vacuum system may have a leak..I would using a vacuum guage inspect all vacuum lines..the PCV hoses are usually in need of replacing after 10 yrs.. low vacuum will reduce the power to move these vac operated door actuators.

with this weather the hvac is in the heat mode always..