Thread: A/C air getting "stuck" behind the dash

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  1. #1 A/C air getting "stuck" behind the dash 
    SE Level Member
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    Dec 2020
    Huntsville, Alabama
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    Hi all,

    I've got a '98 Pontiac Grand Prix GTP with the automatic CJ2 climate control system and a strange A/C issue. It's done this since I've had the car, several years, and I never could figure it out. The A/C otherwise works perfectly.

    So the issue is, on a hot day when running the A/C, for the first 1-2 hours it works perfectly, but then at some point the air seems to get "lost" or "stuck" behind the dash. Airflow from the vents is reduced to a trickle, and I can hear a loud, restricted turbulent airflow blowing (not whistling) sound behind the dash, as if the air is getting trapped or forced out somewhere back there instead. Not enough flow remains to keep the car cool, even at max speed and recirc.

    First thought was blower motor, but the sound and speed of the blower doesn't seem to change at all when this happens, and the full range of blower speed settings still works and produces a change in the loudness of the blowing sound and in the amount of what little airflow is still coming out the vents.

    Next thought was the vacuum-driven mode selectors, but when this happens the selectors still work (and don't fix the problem): I can switch to floor vents, air stops coming from the panel, the same small trickle of air now comes from the floor vents, and most of the flow is still "stuck". Switching back, same thing.

    Similarly recirc mode works but doesn't fix the problem; the blowing sound gets even louder and the trickle of air gets very slightly stronger.

    I also don't think it's the electric temperature blend doors, as I replaced both the driver and passenger side blend doors, and also they work fine when this happens - you can still change temperature and feel the change in the trickle of air.

    I also replaced the vacuum reservoir in the passenger footwell. Didn't fix it.

    What DOES fix it is turning the car off for a while. Too little time and it doesn't fix it, but if I leave it off for maybe 30 minutes or more, it then works again.

    Also, hot air in winter works fine no matter how many hours it runs for. Only cold air in summer causes this behavior.

    The fact that it takes time to start and stop working and that it's only with cold air seems like a thermal effect, like condensation or ice buildup, but I can't think of how that would work. It's not cold enough for ice, right? And the blowing sound does not sound like a gurgling or bubbling of liquid water.

    Could it be that rather than getting "stuck" trying to output air, an intake is getting clogged somehow so it's unable to draw in air? But then how come recirc doesn't fix it?

    If anyone has encountered this or has any ideas, I'd really appreciate your thoughts. Thank you!

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  2. #2 Re: A/C air getting "stuck" behind the dash 
    GrandPrix Junkie
    Join Date
    Oct 2009
    St. Louis
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    Have you put a set a gauges on the A/C system and checked pressures? Maybe there is to low of pressure and the evaporator is freezing up.
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