Check you're car for a CEL (Check Engine Light).
If you have one check you're Trouble Codes.
If you are not throwing a code:
a. You're Idle Air Control Valve on the back of you're intake manifold is bad/dirty.
b. You're throttle body has a fast idle thermo valve & it needs to be overhauled.
c. You have a vacume leak somewhere.
Good Luck.
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