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1977-1982 Is There Always Hot Air Blowing Out?

B

bmotojoe

Guest
Is there always hot air blowing through your heater box? No matter what you do to resolve this problem it is always there? On the 1977 through 1982 Corvettes equipped with and I think without Air Conditioning there are two major components involved. There is the inline vacuum operated hot water shutoff valve that feeds hot water to the heater core and the vacuum shutoff valve that controls it. The inline hot water valve works as a ball valve when you move the heater control slide to the cold position the cable extends and in doing so moves an arm that pushes in the plunger spool of the vacuum shutoff valve sending vacuum to the inline hot water shutoff valve (The Vacuum Shutoff Valve Is Located On Top Of The Heaterbox). Over time the “O” rings on the spool dry out and vacuum bleeds by to the exhaust port of the vacuum shutoff valve never allowing enough vacuum to reach the inline hot water shutoff valve.
There are replacement valves available through most of the major vendors out there but the spring installed in them is to heavy for this applacation. They may work fine for 1969 through 1972 Corvette windshield wiper arm control valve but if used on the heater slide control side the spring is to heavy and the heater control slide will not stay in the cold position. After testing the inline heater control valve with a vacuum pump to make sure it held vacuum and was mechanically sound I removed the old spring from my old valve and after removing the heavy spring from the new valve I installed my old spring in the new valve. Once the vacuum shutoff valve was removed from the car it is a 10 minute repair.
Attached you will see how I did it.
Hope this helps,
Brian

View attachment 13627
 

LVMYVT76

Well-known member
Joined
Apr 21, 2006
Messages
664
Location
springfield, mo
Corvette
1976 medium blue
There are other reasons that hot air might be coming in. The seals on the passenger side side vent may be rotted off and not sealing. Seals around the heater box may be gone. Sealant between the 2 heater box halves may be gone. The seal on the back of the hood may be gone allowing hot engine air to be sucked into the fresh air coming in through the a/c system. These are a few that I have found while working on a/c systems to correct problems.
 

LanceB

Well-known member
Joined
Jul 18, 2003
Messages
455
Location
Louisville
Corvette
1981
So Brian, you don't have "hot feet" anymore? I am willing to cut off my pinky finger to stop this. my wife won't even drive in the Vette with me in the summer because it is "too hot". The bottom vents blow hot air out like a hair dryer, all of the time.
 

Garagetoyz

Well-known member
Joined
Sep 20, 2005
Messages
53
Location
New Jersey
Corvette
78 Silver Anniversary 4 spd Crate engine
Even tho I have closed off the manual valve to my heater core and unplugged the fan motor my feet remain toast. I hope this thread produces an answer to our problem:)
 
B

bmotojoe

Guest
No more hot feet! My fan is designed to run on low speed when the fan switch is in the first position. You would have had to have a softball size hole in the center of the firewall to have that much hot air blowing in from the engine compartment. I know I didn’t so after looking at the assembly manual vacuum schematic discovered this valve. The replacement valve bolts in directly but the spring is way too heavy to keep the slider arm in the cold position no matter how I adjusted the cable. So that’s when I came up with this repair tip and I thought I would share it with others with hot air blowing in through the heater core. If you’re having the same problem I suggest first checking the vacuum pressure at the hose of the vacuum valve in the heater hose. If there is no vacuum or weak vacuum I would replace the vacuum valve on top of the heater box.
A vacuum gauge or even better gauge w/pump is a must tool for our C3’s. IMO.
Brian
 
B

bmotojoe

Guest
This repair tip was preformed on a totally stock engine and exhaust setup. If your running headers you may have to install aluminized under carpet padding found at most of your hardware super stores. If hot water is passing through your heater core then your manual valve is not holding.
Brian
 
B

bmotojoe

Guest
In a previous post the manual valve I think they were talking about was added on using a ball or gate style valve replacing the vacuum operated valve in the heater hose line located beneath the overflow reservoir. When closed it will definitely cut the flow of hot water off to the heater core but there are a couple disadvantages to this also, one being if you need heat to the interior cabin you have to manually open the valve also, you will have to remember to open the valve periodically so water passes through the heater core. If closing off the input side hose with the valve over time will let the heater core dry out and it will begin to decay from the inside out.
Brian
 

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