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This sounds like you may have a stuck open Idle Air Control Valve. This part is located on the throttle body, it allows air to bypass the throttle plate when it's shut. The idle air control valve operates the car's RMP's when the gas is not pushed (Throttle plate closed).
The IAC motor is located on the throttle body.
Remove air duct and air resonator box at throttle body.
Disconnect electrical connector from IAC motor.
Remove two mounting bolts (screws).
Remove IAC motor from throttle body.
Install IAC motor to throttle body.
Install and tighten two mounting bolts (screws) to 7 Nm (60 inch lbs.) torque.
Install electrical connector.
Install air cleaner duct/air box to throttle body.