Hi,
On vehicles, there are blend air doors also known as air mix doors. On most vehicles, the actuators for these doors are electric. On your vehicle, it is vacuum actuated. So, we may simply have a vacuum leak. Have you ever noticed if the temp is more likely to change when the engine is under a heavier load? Also, with the engine running, have you ever noticed a hissing sound?
I attached directions below for the replacement of the actuator. Check to see if there are any vacuum leaks to it. If there aren't, chances are it needs to be replaced.
Let me know if this helps or if you have questions.
Take care,
Joe
See pics below.
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Sunday, July 2nd, 2023 AT 9:44 PM