77 cabin too hot

[Frank K.]

What can be done in a non air car when the cabin is to hot due to a new heater core

[Mike T.]

Re: 77 cabin to hot

Frank - Do non-Air 77's have the same heater setup as Midyears where there is no shutoff valve to the heater core and where they depend on damper doors to prevent hot air from coming into the compartment? If so, maybe the foam seals needed to be replaced in the heater box?
Some guys have added a non-original shutoff valve to the hose feeding the heater core to prevent that problem.

[Larry M.]

Re: 77 cabin to hot

Frank - Do non-Air 77's have the same heater setup as Midyears where there is no shutoff valve to the heater core and where they depend on damper doors to prevent hot air from coming into the compartment? If so, maybe the foam seals needed to be replaced in the heater box?
Some guys have added a non-original shutoff valve to the hose feeding the heater core to prevent that problem.

Good advice. I believe that some of the late 1970's car did have a shutoff valve in the hot water supply to the heater, except it was a fail open valve (heat) versus a failed closed valve (cool) like the midyears. If you have this valve check its operation. If you don't, consider installing.

I would also check either the cables or vacuum hoses that operate the heater box internal doors. A mis-adjusted cable or cracked hose could also be an issue.

Larry

[Joe L.]

Re: 77 cabin too hot

What can be done in a non air car when the cabin is to hot due to a new heater core

Frank------


There is a valve in the system in your car. Yours may be inoperative. Currently, it's available from GM under GM #12515169. Replacement procedures should be covered in your factory service manual.

[Tom R.]

Re: 77 cabin too hot

What can be done in a non air car when the cabin is too hot due to a new heater core

If this is happening after the installation of a new heater core, in addition to the water shut-off valve being a suspect, I would also consider that the vacuum line that operates the actuator activated plenum door would be suspect as well. Your heater/air console control panel (switch) directs vacuum to those plenum doors to open and/or shut the door. If the door isn't shut, it allows heated air into the cabin.

The switch routes vacuum to the plenum doors, subsequently directing hot air flow to dash vents, floor vents or the windshield vent. A Bowden-type sheathed cable (mounts on the top of the box) connects the console control switch to each vacuum actuator. It’s this vacuum actuator that opens and closes a door, routing heated or cooled air through the ductwork to the selected cabin vent. Only the blower motor is electrically operated...all other controls are operated by vacuum.

To troubleshoot, a vacuum pump and gauge are helpful to see if the circuit is holding vacuum; does the door open/close with vacuum. With the gauge and pump you can troubleshoot without running the motor.

Having said all that, it would be easier to determine if a shut-off valve is the fix since it is easily accessible from the passenger side engine compartment. Note that after model 1975, both A/C and non-A/C Corvettes are installed with a water shut-off valve. All Corvettes from 1976-79 are equipped with GM# 495613 water shut-off valve. In 1979, 495613 was replaced with GM# 14020087 and used through 1981 production. the number Joe lists above may have superceded these part #s.

The valve mounts in the heater hose inlet circuit that enters the firewall on the passenger side and is activated by vacuum when A/C controls are set to “MAX.” The notion is that hot water circulation is prevented once the valve closes, thus keeping heated coolant from entering the heater core and heat transfer to the re-circulated cooled cabin air. But C3s were known to "leach" hot air into the cabin and therefore plenum doors must be trouble shot as well. An excellent book on the topic, now out of print is "Air Conditioning Strategies for the 63-82 Corvette" by Michael J. Davis.

[Monte M.]

Re: 77 cabin too hot

Frank,
The guys have given some great advise to hopefully remedy your problem. can I ask why you say it has to do with a new heater core? Did it start when you put in a new heater core/

If this is the case, did you make sure all the gaskets and seals to the firewall and heater box were all still in good shape. It is very difficult to try to reuse the old one and not replace them. In changing out the heater cores, one of the steps, depending on the car, is pulling the carpet back a bit to have room to access where you need to get to. If by chance you did pull the carpet back, did you remove the carpet plugs, and put them back?

Just these small holes being open will allow a lot of heat to come into the passenger compartment. If one of the heater box seals is not exactly where it should be can allow a lot of heat to enter the passenger compartment as well.

These are just ideas that may or may not apply to your car, but the heat has to be coming from somewhere. Over the years I have owned quite a few older Corvettes. When they are hot inside, which they are quite often, these are the first places I typically look and find the problem. One is easy to fix, while the other takes a bit of time.

I too have the same issue on my 72, big block, A/C car. I replaced the heater core and the heater box gasket did not seal right. I had a ton of heat coming into the car until I installed a hot water valve. And no, I have not gotten around to fixing it yet. A big block with A/C takes quite a bit of effort to tackle again. It is high on my list of things to do though.

?H?ope this helps.

[Joe L.]

Re: 77 cabin too hot

One correction: I failed to note that the 1977 involved here is a NON-A/C car. I believe the valve I mentioned above is for cars with A/C. I can't find a valve for 1977's without A/C but, if there was one, it's possible it's the same as for cars with A/C.

[Frank K.]

Re: 77 cabin too hot

Thanks! I need a gate valve to shut the heat off completely. I remember seeing a vendor advertise such a valve. Does any one know a source? Thanks

[Monte M.]

Re: 77 cabin too hot

Frank,
Before doing that I think I would remove the passenger side of the dash and do as Tom mentioned. With that side of the dash off you should be able to get your hand on the actuator or the door lever itself to see if it is closing when it is supposed to. This is the easiest way to check to see if everything is working correctly.

If that is working correctly, there has to be a seal failure somewhere on the heater box. If all else fails and you cannot resolve the problem, then put a water shut off valve in.

If you have a seal failure, such as I do on my car where the air is coming into the heater box from the engine side of the firewall, you have to stop that air from coming in. I used a little dope type product to stop the air coming in from the engine compartment on mine. I have decided to wait until the motor is out of mine to fix it the right way.

I won a bet with my son. He has to pull the motor out and clean the engine bay. I figure it will be a lot easier to fut another set of seals in at that point rather than fight it now.

With the car being a non-A/C small block, it should be very easy to change out the seals.
An easy way to test to see if the heater box is sealed or not is smoke. You should not have to have the car running to do this. Without the blower motor fan running put your smoke under the heater box area inside the engine compartment. Closing the hood will help out quite a bit. No open flames, of course.

If that does not work, turn the blower motor on high. See if that clears the smoke away from the engine side of the box. If smoke comes in the passenger compartment, or the fan blows the smoke away from the heater box, a water valve will not help you out. You are going to have to take the time to replace the seals on the heater box. This is one time to be happy you do not have A/C.

Give us some more information. What have you tried? How did all this happen? When are you experiencing the heat coming in the passenger compartment? Whatever you can give us will help us resolve this issue with you.

Best of luck,

[Michael B.]

Re: 77 cabin too hot

Thanks! I need a gate valve to shut the heat off completely. I remember seeing a vendor advertise such a valve. Does any one know a source? Thanks

Frank

I have the same configuration. You want part

http://www.parts123.com/corvettecentral/dyndetail.pta?catalog=0000050g&ukey=12984

In fact when I had overheat problems before I restored out 77 I used this to help cool down the car when stuck in traffic because it would divert so much to the heater core. The other suggestions for adjusting the flap in the box and changing the flap material (I used a rebuild kit with neoprene pad) will help but if this valve is stuck open no amount of adjustment to the heater box will help.

If the problem only started after the heater core was replaced it's because the heater core was plugged up and your valve was already stuck open (or bypassed).

Hope that helps.

Michael B.

[James B.]

Re: 77 cabin too hot

Frank,
The guys have given some great advise to hopefully remedy your problem. can I ask why you say it has to do with a new heater core? Did it start when you put in a new heater core/

If this is the case, did you make sure all the gaskets and seals to the firewall and heater box were all still in good shape. It is very difficult to try to reuse the old one and not replace them. In changing out the heater cores, one of the steps, depending on the car, is pulling the carpet back a bit to have room to access where you need to get to. If by chance you did pull the carpet back, did you remove the carpet plugs, and put them back?

Just these small holes being open will allow a lot of heat to come into the passenger compartment. If one of the heater box seals is not exactly where it should be can allow a lot of heat to enter the passenger compartment as well.

These are just ideas that may or may not apply to your car, but the heat has to be coming from somewhere. Over the years I have owned quite a few older Corvettes. When they are hot inside, which they are quite often, these are the first places I typically look and find the problem. One is easy to fix, while the other takes a bit of time.

I too have the same issue on my 72, big block, A/C car. I replaced the heater core and the heater box gasket did not seal right. I had a ton of heat coming into the car until I installed a hot water valve. And no, I have not gotten around to fixing it yet. A big block with A/C takes quite a bit of effort to tackle again. It is high on my list of things to do though.

?H?ope this helps.

Monte,

What specific hot water valve did you install?


Jim