Could be a lot of reasons. First things to check would be to look for loose vacuum lines or other vacuum leaks (like where the intake manifold bolts to the engine).
You could also have a defective Idle Air Control valve (IAC valve).
You could also run a computer scan to see if any code comes up related to things like EGR or O2 sensors, or crankshaft or camshaft or cooling temperature sensors.
Idle speed is adjustable on your car but it is somewhat critical and should probably be done by a professional technician.
I identified a chewed up small hose after hooking up a work light today as I was preparing to get to the bottom of idle problem. I'm not sure what its function is but I repaired it and idle while running in park is at 750, down from 1500. What did I do right? And thank you for helping me. I'm single, living in the country. I also found a huge mouse house under manifold!
You plugged a vacuum leak, I would guess---and yes, mice often do things like this in the country. Maybe setting traps if you have a garage or sprinkling some offensive material under the car might help (moth balls?)
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You could also have a defective Idle Air Control valve (IAC valve).
You could also run a computer scan to see if any code comes up related to things like EGR or O2 sensors, or crankshaft or camshaft or cooling temperature sensors.
Idle speed is adjustable on your car but it is somewhat critical and should probably be done by a professional technician.