I have a vacuum leak coming from the temp control. I hear a hissing sound. also Poor air is blowing either on AC or heat. My problem is primarily with the heater. It isnt blowing hot air. When set on air vent coming from dash as i do a hard acceleration the air switches and blows thru the defroster only. on normal driving it shifts intermittently between dash vents and defroster. i have replaced temp control switch. no luck. Any suggestions? Or fix? Do i have to replace heater core? Since i am not getting hot air? I have tested and felt both hoses from the heater core and they are both hot. I read if one hose is hot and the other cold then the heater core is clogged and has a problem. Help!

 

ok....do you still have a vacuum leak or not? you said that you had one and then you said that you replaced the part, so where do you stand with that?

i had an 89 pick up (same dash) and mine leaked too, but i never replaced. i just redid all the vacuum lines in the engine bay when i had the motor out one time. it didnt fix my problem because the selector switch is what was actually leaking but it did help because i was then getting enough vacuum at the switch that when it did change over, it would stay there. so if you have replaced the selector switch, try looking for a vacuum leak either further down the line, like by the blend door, or looking under the hood. you may not be getting enough to the switch to begin with and thats the noise that you are hearing. its changing part way and doesnt have enough to complete the task.

 

No. I still have the vacuum leak coming from the selector switch, forgot to mention ive been having this leak years ago but was not affecting blowing of air to dash vent and defroster intermittents. It just got worse now and its not blowing hot air. I dont know where to look under the hood! Should i also be looking for hose leaks behind the dash? Thanks

 

How do i solve the warm air and not hot air that i should be getting. and also poor air blowing in general. I noticed on the passenger side vents is blowing only little air.

 

check under the dash to see if you have something blocking the blower intake. something like a plastic bag or piece of paper. as far as not having hot air, i never got the chance to drive my truck in cold enough weather here in southern cali to even try the heater.
good luck
-Jake