Hi all,
I have had a number of old C3s and the heater - A/C- Vent system is a subject of a lot of heat issues.
Pre 77 Corvettes had no heater shut off (full time hot water in the heater core) and will benefit from a manual heater shut off in the heater hose. The heater core is just blocked by a Flapper /plate to vent freash air into the car (vent and A/C mode) or thru the heater core for heat. The Flapper has foam seals that go bad thru the years and at best only minimizes the heat transfer. So having a manual shut off stops a lot of that heat from coming in. That gets rid of a couple of quarts of hot water just setting in you dash!
The later 77-82 vetts use a vacuum shut off that turns the water flow off when vacuum is applied and therefore it is open when the engine is shut off or vacuum is low. So a bad vacuum from the control valve, or from the AC control can also cause a lot of heat to come in when on vent or A/C setting. So if you are getting a lot of heat in the cabin then you need to check all that too. A manual valve in the hose can also solve that problem.
Need to check the flap control (manual cable hot-to-cold lever) and make sure it is closing all the way and also make sure the heater core is not getting hot water because of bad controls or stuck valves.
There is a lot to check to make sure it is all working right! But when the corvettes get 20+ years old, the rubber, valves, vacuum diaframs and such all get stiff, rusty, worn and need to be checked and/or replaced!
Good luck! Have fun!
Gary
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