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Q: losse of heat when going 70.

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The heat works great up until around hitting 70mph, then it starts blowing cool air. The temp guage also drops. If you slow down, it goes back up and heat returns. Antifreeze is full.. Temp guage???

My car has 124000 miles.
My car has an automatic transmission.

Hi there. I have come across this same problem several times. The majority of the time it is caused by a failing thermostat. Due to the poor design of a metal thermostat molded into a plastic housing these will not wear the same and the thermostat ends up sticking in the housing and no longer closing correctly. If this occurs, the coolant does not stay in the engine long enough to get hot, and will get hot under an increased load, such as going up a hill. I would typically change the thermostat first. If you needed to have this done, consider YourMechanic, as a certified mechanic can come to your home or office to replace the thermostat for you.

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