help, vent blows warm air

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o k so i have an 89 crown vic that when the temp bar is all the way left the vents are supposed to blow cold air. they do except when i drive up a hill, then they blow HOT air, can it be a vaccum issue ? this is driving me nuts... mike

[KimMG]

When was the last time you had your a/c serviced?

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kimMG, it has been a while since ac was serviced, but the electric is unpluged from the compressor. vent position on htr control should be outside air (?) only. i have a hunch it is the "blend" door. i think it is controlled by vacuum. so where does vacuum signal come from ? mike

[ModMech]

It sounds like the vacuum check valve for the blend door is no longer a check valve.

This allows the vacuum to drop with manifold vacuum, which can cause the blend door to move to the warm position at low manifold vacuum.

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modmec, thanks for the info. where is it located ? is this something i can do without pulling the dash ? mike

[way2old]

There should be a small plastic valve in the line right above the gas pedal just after it comes through the firewall. Remove it and try to blow through it both ways. Air should only flow one way. If it works properly, remove the controls and look to see if a vacuum line is loose behind the heater controls.

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way2old, i looked above the gas pedal area on the firewall. could not see where a vacuum line comes thru. checked engine side of firewall and could not find any sign of a vacuum line in that area entering cabin.
checked all the vacuum lines to the controls and they seem tight.
lots of different colors. is one a key to this 1 way valve ? mike

[bruzer321]

I have this same problem, I cant figure it out this time, I had it once before and it was the "coffee can"/vaccum canister, it had a rust hole in it by the mount, i bought a new one for $27 at the ford dealer and problem solved but this time i have no idea what it is, it blows freezing cold at idle (still have R-12) and then on acceleration or load it blows hot

[bruzer321]

i found my problem and fixed it, i have automatic climate control and there are two vacumme lines that plug into my coolant lock out or whatever it is, its something that goes inline to one of the hoses for the heater core its right by the firewall right next to you evap/drier for your a/c, well those two lines somehow became unplugged, plugged them back it problem is gone now i just think i need to charge the system up some, the air isnt as cold as i would like it