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  1. #1
    New Member markwrob's Avatar
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    Corvette(s)
    1981 L81

    Default Heat from under dash???

    Hello,
    This is my first post, forgive me if this has been asked before. I just got my '81 couple of weeks ago and I noticed that I'm getting a lot of heat from underneath the dash or somewhere in that area. It makes for a very uncomfortable drive, especially on hot days. All climate controls appear to be working? Anyone else had this?

    best regards,

    Mark W

  2. #2
    Member ladams's Avatar
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    Corvette(s)
    1981 White

    Default

    Quote Originally Posted by markwrob View Post
    Hello,
    This is my first post, forgive me if this has been asked before. I just got my '81 couple of weeks ago and I noticed that I'm getting a lot of heat from underneath the dash or somewhere in that area. It makes for a very uncomfortable drive, especially on hot days. All climate controls appear to be working? Anyone else had this?

    best regards,

    Mark W
    Same problem with my '81, but I think mine is just a case of not having heater controls as the vacuum system is all wonky. Someone suggested a quick fix of unplugging the heater fan to me. I haven't tried it though. I'm still working on a brake problem.

  3. #3
    Member wishuwerehere82's Avatar
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    Corvette(s)
    Red '82 Coupe,Sebring Silver '98 Coupe

    Default

    Your 81 is equipped with Astro-ventilation, which means even though your fan blower is off it still blows a little bit of air through the ducts.
    If your heater/AC/vent/defrost controls seem to be working right and yout fan speed selector switches the fan speed OK, then the problem might be in the vacuum operated switch that controls the flow of coolant through the heater core. It is located in the engine compartment between the heater hose and the air box. There is a vacuum line that goes to the top of a cylindrical actuator that opens the valve when vacuum is applied to it.
    When it opens, the coolant is allowed to flow from the water pump through the heater core and then the coolant returns to the intake manifold.
    It looks like this.

    They are known to stick open and if they do, the hot anti-freeze will keep circulating into the heater core. If it's sticking, hot air will be coming out your A/C vents all the time regardless of whether you move the control to cool or not.

    If that's not the problem, then I have to tell you that the C3's have a lot of heat transfer in the transmission tunnel area. Keeps your knees real warm. Not much you can do about it unless you pull up the carpet and insulate the tunnel hump.
    Pete

  4. #4
    Member ladams's Avatar
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    Quote Originally Posted by wishuwerehere82 View Post
    Your 81 is equipped with Astro-ventilation, which means even though your fan blower is off it still blows a little bit of air through the ducts.
    If your heater/AC/vent/defrost controls seem to be working right and yout fan speed selector switches the fan speed OK, then the problem might be in the vacuum operated switch that controls the flow of coolant through the heater core. It is located in the engine compartment between the heater hose and the air box. There is a vacuum line that goes to the top of a cylindrical actuator that opens the valve when vacuum is applied to it.
    When it opens, the coolant is allowed to flow from the water pump through the heater core and then the coolant returns to the intake manifold.
    It looks like this.

    They are known to stick open and if they do, the hot anti-freeze will keep circulating into the heater core. If it's sticking, hot air will be coming out your A/C vents all the time regardless of whether you move the control to cool or not.

    If that's not the problem, then I have to tell you that the C3's have a lot of heat transfer in the transmission tunnel area. Keeps your knees real warm. Not much you can do about it unless you pull up the carpet and insulate the tunnel hump.
    Excellent info., thanks (even though this is not my thread) haha, where does the vacuum line lead too?, Also, how about the directional controls? Are they vacuum operated too? (ie. defrost, floor, vent)

  5. #5
    Member wishuwerehere82's Avatar
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    Red '82 Coupe,Sebring Silver '98 Coupe

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    Quote Originally Posted by ladams View Post
    Excellent info., thanks (even though this is not my thread) haha, where does the vacuum line lead too?, Also, how about the directional controls? Are they vacuum operated too? (ie. defrost, floor, vent)
    Everything except the fan speed control and A/C compressor is Vacuum operated and controlled.
    The heater valve is connected through a valve on the selector switch. One line goes to the heater valve, the others go to the diverter door actuator on the heater box.
    Having a good vacuum connection to the intake manifold is essential for proper operation of the air flow controls.
    There is a vacuum switch as well as an electrical switch on the selector. They do get brittle after time and temp. get to them, and the nipples that the vacuum lines connect to can break off. A constant hissing sound in the console is a tipoff that one of the input connections is broken. Hissing in just one function is a broken output connection.
    Respectively, if you hear a hissing sound around the heater box, then the vacuum line or actuator diaphram is broken there.

  6. #6
    Member petes74ttop's Avatar
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    Corvette(s)
    1974 T-Top

    Default Flash Back!

    Bought my 74 T-Top back in Nov 2000, the PO had replaced the heater hoses but left off the control valve, talk about excessive heat! What did I know? PG.

  7. #7
    New Member markwrob's Avatar
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    1981 L81

    Default

    Thanks everyone for the quick replies, at least I know where to begin. I really appreciate the help. Is there a way to test the hot water valve? There is nothing on that in the shop manual???

  8. #8
    Moderator bill81vette's Avatar
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    1981 dark blue metallic

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    WOW!!! Thanks for the info Pete
    now I know why mine is so hot as well!!!!
    I thought it was just from the engine even though I put in a heat barrier to help cut down on the heat transfer from the engine comp.
    Bill and Gail...save the wave
    NCRS member 42415
    1981 St.Louis car #2735
    all matching numbers

    born Sept 18,1980
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    Without Trucks......
    You would be Hungry,Homeless and Naked!!!!!!



  9. #9
    Member wishuwerehere82's Avatar
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    Red '82 Coupe,Sebring Silver '98 Coupe

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    Quote Originally Posted by markwrob View Post
    Thanks everyone for the quick replies, at least I know where to begin. I really appreciate the help. Is there a way to test the hot water valve? There is nothing on that in the shop manual???
    One way, the quick and dirty way is to feel the hoses going in and coming out of the valve.
    Turn off the heat, start the engine, and feel the hoses for temperature as the engine warms up.
    If it is sticking, both hoses will feel hot to the touch, in fact the same temp as the radiator hoses.
    If it is closing as it should, the input hose from the water pump will be cooler than the radiator hoses since no coolant is flowing through the valve. And the output side to the heater core should be relatively cool to the touch.

    The other way, a little more involved, is to connect a new hose directly from the water pump to the nipple on the intake manifold. Thus bypassing the valve, heater core and the whole shebang. This is just for testing purposes of course.
    If it is noticably cooler after doing this, then it's a good bet the valve is the culprit.
    ( Like I should talk, since my heater core has been bypassed for 10 years, but then I live in a cold state except for this week.)

  10. #10
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    Corvette(s)
    1981

    Default

    I want to chime in here if this is ok. I have a bad heater core and I bypassed it by looping the hose. My "hot feet problem" was gone as well. Great to drive now. My question is, can the A/C or fan work once this is bypassed? Or is it all or none for climate control when the heater core is out of the picture? Plan on replacing heater core but don't have the guts to tackle at the moment.

    Lance

  11. #11
    Member petes74ttop's Avatar
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    1974 T-Top

    Default It will be cooler than ever!

    Hi LanceB, yes, your fan and A/C will work fine, maybe better than ever.
    Replacing the heater core takes a lot of steps, but with the help you'll get on this forum it will be a piece of cake, PG.

  12. #12
    New Member markwrob's Avatar
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    Corvette(s)
    1981 L81

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    Thanks again for the help. So the hoses feel very hot on both sides of the valve when the engine warms up, so I assume the valve is stuck open. But with that said, I pulled the vacuum hose to see what happens and my headlights pop-up, that leads me to believe maybe there's some vacuum lines mixed up or something ??? Does anyone one know a website where I can find a vacuum diagram, the only ones I'm finding when I do a google search dont have the water valve in them??? I cant find anything in the shop manual either.

    best regards,
    Mark

  13. #13
    Member ladams's Avatar
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    Quote Originally Posted by markwrob View Post
    Thanks again for the help. So the hoses feel very hot on both sides of the valve when the engine warms up, so I assume the valve is stuck open. But with that said, I pulled the vacuum hose to see what happens and my headlights pop-up, that leads me to believe maybe there's some vacuum lines mixed up or something ??? Does anyone one know a website where I can find a vacuum diagram, the only ones I'm finding when I do a google search dont have the water valve in them??? I cant find anything in the shop manual either.

    best regards,
    Mark
    Hey Markwrob,
    Please let me know if you get any sort of diagrams as I have some of the same problems as you.
    Thanks

  14. #14
    New Member markwrob's Avatar
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    I got a picture or two but can't make out what's what??? I will try to post some scans later on. I ordered a valve anyway, it seems inexpensive so I might as well give it a shot before I dive into the vacuum hell....

  15. #15
    Member ladams's Avatar
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    Quote Originally Posted by markwrob View Post
    I got a picture or two but can't make out what's what??? I will try to post some scans later on. I ordered a valve anyway, it seems inexpensive so I might as well give it a shot before I dive into the vacuum hell....
    Yeah,
    I look on ecklers and corvette central, etc, but they only show flow chart looking things instead of actual shapes or names of parts.

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