[Razorback1980]

Still seems weird though the valve would make it overheat. I don't think putting the original valve back in there will stop that from happening. You could until you get this figured out bypass the heater core altogether. But the key is you need to figure out why it's overheating when the valve is closed as I don't think this should happen. I've never had a heater control unit apart so I don't know if the amount of heat coming into the car is controlled by an air flap or the amount of hot water flowing through the heater core. If it's controlled by the amount of hot water going through the heater core, this definitely shouldn't happen no matter which valve is in place. I don't have an early model and I'm not sure if the laters are different designs or not. I'll have to check into the manual and see how the coolant flows but usually the heater is a secondary path and a primary path cools the engine.