plutexams

The heater just blows cold air most of the time, although there are times it is slightly warm. I have replaced the thermostat and the temperature gauge is steady where it should be (about 180 Degrees). Sometimes if I step on it there will be warm air briefly but then it reverts to cold. It is not dependent on RPM, I have run at 3000 RPM with no change in heat from the heater. It doesn't matter which position the heater controls are set at and those controls seem to work. When set to defrost I get air out of the vents pointing at the windshield, when set to vent air comes out the dash vents. If the temp gauge was not steady and the thermostat replaced I would have guessed it had a bad thermostat. Any help would be appreciated.

speedreed8

There is a vacuum controlled water valve in the heater hose line. It probably is stuck closed or is leaking by a small amount. May need to be replaced.

Lagonia

Close to the passenger fender wall in the engine bay - easy to spot. Disconnect the vacuum line and hook up a vacuum pump - pump and see if the lever is moving. If not, its toast. If it does, make sure the vacuum line does have vacuum when the temperature rod is turned all the way to the hot range in the temperature control assembly - if you still do not get vacuum, I would suspect the vacuum line routing or the vacuum control element which is located under the center console (part off the temperature control assembly). These tests should be done with the car running (obviously).

derekderek

May well be it. Hitting the gas changes the vac signal. Also, once you have it found and held open, run a garden hose through the heater both directions. Could be plugged up with 40 years of stuff. Don't turn it on hard...

plutexams

Thanks for the suggestion. I found and took off the vacuum control valve and it was not plugged and by putting vacuum on it, it seemed to be working. I noticed that the valve is open in no vacuum position and closed with vacuum applied. I plugged the vacuum line and reinstalled the valve and now have heat. But the doors to open the heat, vent and defrost are working intermittantly at best. I suspect a vacuum problem but haven't been able to find a good diagram of the vacuum system, especially related to the carborater. Any help would be appreciated.

hunt4cleanair

These are always interesting problems given they are vacuum related and troubleshooting is checking the various systems that come together to deliver heat where you expect it.

Once you've confirmed the vacuum control valve is functioning properly, you must then follow the vacuum line into the vehicle.

In 1978, the vacuum travels to what I would call a splitter located behind the glove box if memory serves me correctly then routes to the console climate control panel (heat, A/C, vent, etc). But the system also has vacuum operated actuators that make duct doors open and close, depending on where that switch is set, defrost, heat, air.

If defrost works but floorboard heat doesn't, then you have to look to those vacuum-operated doors behind the kick panels. Now here's where it gets tricky. When the problem is heat related, an actuator is located to the left of the heater box and to access it, you nearly have to remove the glove box and duct work to see it. So, troubleshooting is easy...once you access it. If you take this on, its best to take the passenger seat out to add space to work and remove parts. It's not complicated or difficult, just tedious. Sounds like this is the actuator/door that is not functioning properly!

When I replaced my heater core some ten years ago, I went the extra step to check all those vacuum lines under the dash and then confirmed the actuator doors operated as expected. All this can be done with engine off and using your vacuum gauge/pump.

Hope this helps!

Keep us posted what you find out.

SwampeastMike

The hot water shutoff valve (in the engine compartment) is controlled by the hot water cut-off switch behind the glove box and on top of the plenum.

When you move the temperature control all the way to the left (cold) a lever depresses a button on the cut-off switch. When the button is pressed vacuum is applied to the shutoff valve and it closes preventing hot water from circulating through the heater core.

Since you have constant vacuum at the valve, the switch is either defective or improperly connected. The vacuum lines for the HVAC (including the connection to engine vacuum) are tiny. Any significant leak will cause intermittent (at best) operation. If the cut-off switch is leaking or improperly connected it may well be the only problem.

lvmyvt76

find a corvette club in your are(call the local library for car club lists) and find someone there that might want to help you!! GOOOD luck with your 78, save the wave, WELCOME to the forum!!!