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Discussion Starter · #1 ·
I'm having HVAC issues with my 99. I was driving around town the other night, and I pulled into a place to eat. When I came back out, I pulled out and started driving down the road, and all of a sudden the floor vents started blowing hot air and fogged up my windshield in less than a second! I had my HVAC set to "Off, coldest temp". So I had to put the defrost on, and got the fog to go away. but then I noticed now that the floor vents always air out now when going down the road. I don't think they used to do that. But anyway, i want to know what may have happened to make it blow hot air out the floor vents when the setting was on "Cold", and fans were off. This was at night, about 70 Deg outside.
 

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The blower power goes though th hvac panel. The mode is also controlled by a vacuum switch which is also in the hvac panel. By looking at the wiring diagram you could have a badly shorted out panel or possibly a shorted high blower relay to make the fan run by itself. I would pull out the panel and look for fried wires and broken vac lines. I can't help but think some kind of vacimn leak is causing all your probs by the blower , that's a head scratcher
 

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Discussion Starter · #3 ·
I don't know how I would have shorted something out. I wasnt messing with anything. It hasn't done it since, but I was freaking out thinking I blew a heater core or something, But the passenger side of teh floorboard is dry as can be.

Is there supposed to be air flow from the floor vents when the Hvac is off? I only feel it when I'm moving about 40 or more.
 

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with the blower off you will still get ram air pressure though the cowel, if the blend door is set for heat it will come out hot. I'm not certain what the default position is. My car doesn't do this, when its off its off. I ordinarily run around in vent with the temp dial to cold. there's a vac line that runs though the firewall to give the hvac vacumn. Why not trying unplugging and capping this just for giggles and see if your miss goes away./
 

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Discussion Starter · #5 ·
Where is that line you speak of? Have a picture or a general idea where I'm lookin? I had it set to cold, thats why it's so weird that it blew out hot air for a few seconds. Made no sense to me at all.
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Generally when you get the air flow screwed up, it's either the check valve in the vacuum system, or the cable from the tenperature control to the flapper door that selects between heat and A/C.

HVAC controls, courtesy of Shoebox:

 

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Discussion Starter · #7 ·
Generally when you get the air flow screwed up, it's either the check valve in the vacuum system, or the cable from the tenperature control to the flapper door that selects between heat and A/C.

HVAC controls, courtesy of Shoebox:

I looked this over and looked all around my HVAC controls. I tried to pull it out, but I can't get it out farther than an inch. It's like the temp cable is really tight. How do I release that? Also, The air flow was all the same. The issue is it became extremely hot when coming out of floor vents. It stayed on floor vents the whole time, until I turned on the defroster to get rid of the fog. Once I turned the defroster off (flipping the switch over to "Off"), it went back to coming out the floor vents. I guess my main question is: are you supposed to feel air coming out of the floor vents when the system is set to "off"? And why would it blow hot air from the floor vents when the temp is set to cold, and the system is "Off"?
 

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1994 Firebird Formula 381ci LT1 / TH400+GV O/D
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Check the cable connection on the flapper door. See if it moves when the knob is turned. Don't pull the HVAC panel out.

If you look at the diagram, the only way air does not flow through the heater core is if the temp knob is in the extreme cold position. That moves the door (#6) so the air coming out of the AC evaporator bypasses the heater core. I don't know for sure on the V6 cars, but in the V8 models, there is always coolant flow through the heater core, so the only way you wouldn't feel heat is if the door is moved fully to the position to prevent the incoming air from flowing through the core. If the flapper door as even slightly open, some air is going to flow through the heater core.

Then the vacuum controls which vents it comes out of - floor, dash or defrost.
 

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Discussion Starter · #9 ·
I can hear the flapper door opening and closing. The change in temperature was as if I had turned the knob all the way to hot and turned on "feet" setting on the hvac. I will check that check valve tomorrow and see what the deal is.
 

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Discussion Starter · #10 ·
So, i'm back with the same issue, but with a little more helpful detail. I had gone home after work, and as I was leaving, I started my car, and let it idle for a minute. the car was already completely warmed up by this time because I had just driven 10 miles home from work. As i let it sit at idle, it made a 'rushing water' sound (but very muffled) from behind the dash. I started to drive away, and my windshield fogged up immediately. The floor vent was pushing in HOT air. The HVAC was set to "off" and temperature all the way to the cold side. (by the way, I learned that when the system is set to "off" it will go to the floor and defrost as a default location, told from GM). So after I felt the floor vent letting in hot air, I turned the HVAC on to the far right position, which is defrost. A/C kicked on, Boom, fog was gone. Turned HVAC back off, and this time the floor vent was letting cold air in, and eventually became warm again. I have noticed that my A/C is not COLD, but cool. Does this sound like a flapper door issue, or something else?
 

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Discussion Starter · #11 ·
It really sounds like the evaporator core, or the temperature door, but I'm just double checking here to get any opinions.
 

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Discussion Starter · #13 ·
Coolant's full. Same for the resevior. Always at the same mark. Not losing coolant. I think the fog is just the heat hitting the windshield. I did check underneath the passenger side, but it's dry. There's no coolant leaking outside either.
 

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This sounds to me like the temperature control is not functioning correctly There is a procedure in the 98 service manual regarding the checking and adjustment of the cable. It should be the same for your car. There may be an obstruction keeping the valve from moving as far as designed or the cable may be loose.

The same thing happens to me occasionally (sudden fogging of the windshield) and I am not aware of any defects.
 

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Discussion Starter · #15 ·
This sounds to me like the temperature control is not functioning correctly There is a procedure in the 98 service manual regarding the checking and adjustment of the cable. It should be the same for your car. There may be an obstruction keeping the valve from moving as far as designed or the cable may be loose.

The same thing happens to me occasionally (sudden fogging of the windshield) and I am not aware of any defects.
That makes me feel better. It doesn't happen everytime I drive, but it happens enough to notice something wrong. I will check that service manual for the cable adjustment. Thanks Gary!
I hope I don't have to pull apart the dash to fix this. I already broke one upper dash pad, I don't want to have to break another one.
 

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Ramblin' Wreck
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The control actuator is accessible from underneath the right side of the I/P. Remove the right side lower insulator panel. If you want to look inside the A/C module housing (heater core, etc), there is a large inside cover that can be removed without a lot of difficulty. Remove the glove box also for that process. I did that in about 10 minutes or less. Putting it back, however, was frustrating for me because it was difficult to line up all edges with the main housing.
 

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Discussion Starter · #17 ·
Beautiful Gary, thank you! Have any more pictures of under there? How did you remove the housing shown in the first picture? Also, in the second picture, that little "nipple" right next to the carpet... when the panel is on, there is a clip holding it on. How do you remove that clip? I cannot for the life of me get it off and it's the only thing holding the lower panel on.
 

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If I am remembering it correctly, there are several screws holding the cover to the housing. Very annoying trying to align them on reassembly. I temporarily removed the round actuator in the first photo (behind the orange wire) to get more room for reassembly. It's just slid onto it's holder, no screws). It isn't necessary to remove the cover, I was just curious to see if there were any foreign objects inside.

The push-on retainer? I don't remember; it must not have been too difficult. Probably used a pair of needle-pliers to grip one far edge and twist so that it opens a bit.
 

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Discussion Starter · #19 ·
If I am remembering it correctly, there are several screws holding the cover to the housing. Very annoying trying to align them on reassembly. I temporarily removed the round actuator in the first photo (behind the orange wire) to get more room for reassembly. It's just slid onto it's holder, no screws). It isn't necessary to remove the cover, I was just curious to see if there were any foreign objects inside.

The push-on retainer? I don't remember; it must not have been too difficult. Probably used a pair of needle-pliers to grip one far edge and twist so that it opens a bit.
What is that round thing that's positioned vertically?
 

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