84 C4 Heater Issues
 

Good morning gentlemen. I have a 84 C4 that the heat works when it wants too. The shutoff valve is operating correctly to let fluid flow to the heater core and the vacuum lines are not leaking. Randomly it will work when it wants too, usually once the weather warms up. I’m thinking that there is a crimate controlled vacuum switch that is bad somewhere under the dash, but wanted your inputs before I tear into it. Any help would be greatly appreciated.

I had a similar problem. Try flushing the heater core. Move the surge tank disconnect the heater hoses at the fire wall and hold a garden hose over each pipe. Go back and forth until water runs clean.

I would think that it was Hester core related, but that’s new as well. Any other thoughts?

Even a slight amount of fluid down will cause your issue. Make sure there is no air and full.

I would use a quality system flushing chemical in the cooling system and follow the instructions on the bottle. I use BG's Super Flush and its been a great product for me. I would then attempt to back flush the heater core by removing both lines going into the heater core and flushing water backwards through the heater core and see what comes out of it after the flush loosens up some of the sediment that may have accumulated in the heater core. Flush it backward and forward until its clean and clear. I would remove the block drain plugs and flush the engine and do the same for the radiator and install a new thermostat and gasket. This should give you a good base to start with, knowing you cooling system is clean.

Thanks for the inputs gentlemen. I will flush it and change the thermostat today, but I don’t think that is the issue. I’ve been turning wrenches for years and this one honestly has me stumped. Hope you all have a great day.

Mine was doing crazy stuff. Sent the Blend door motor, Programmer, and control to Batee for rebuild. All is well again.

Its possible that you have a vacuum issue causing the blend door to malfunction. Perhaps the blend door is binding and it will unstick with temperature change.

The blend door opens electronically but the vac programmer has something to do with that on the 89.

Finally fixed it
 

Update on the Vette. After flushing everything it still didn’t fix the problem. Dug into the system and all of the doors were actuating right. After more troubleshooting, it ended up being the plastic clip that attaches to the flap door had broken. I couldn’t see it until I unhooked the temp control cable and unbolted the assembly. Hope this saves someone else some time later on. Thanks again for all of the advice.

Glad you were able to find the issue and resolve it. Nice to know that this advice is out there now for this potential issue. And not a bad thing that you have fresh coolant in the system!