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Heat turns to cool air on the highway. 69 GTX convertible.

Bb69gts

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FBBO Gold Member
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Hello forum members. new at posting, so please be patient.
I have rebuilt my 69 GTX convertible, AC car. Everything works as expected, A/C, Heat, Defrost and blower, but as we get to highway speeds, say 60 MPH and above, my heat quickly turns to cool air.
I don't know if I am losing vacuum to one of the Heater box pods, or the airflow is forcing open a vent door, but the heat stops working. I do not believe the cool air is mixing with the hot air and becoming cool. I think the hot air quits working.
Anyone have any suggestions.
Brad-TX
 
If it’s all fresh, check your coolant level again, if it’s really low I think that can happen
 
If it’s all fresh, check your coolant level again, if it’s really low I think that can happen
Thanks. operating temp is dead solid 190 this time of year. rarely moves. It does get much hotter in the heat here in TX, but not losing temp which i agree could cause the Heater core to be cut off by the firewall heater control valve.
Just to answer directly, the coolant level is not low and I haven't seen any signs of leakage.
Thanks
 
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