You have a vacuum leak most likely. Or your one of your O2 sensors could be bad. Start by grabbing some throttle body cleaner and spraying around the manifold, listen for idle changes. Its usually the manifold gaskets. When replacing, replace ALL of them, front, back, bottom, CCV or OSV (OEM ONLY!), and one of the most ignored vacuum leaks are the fuel injector o-rings. Its a HUGE importance that the intake manifold be torqued down in an X-pattern, making sure the same pressure is placed on the gaskets so the are seated right. Also make sure the gaskets are sitting on a shinny clean surface, use brake cleaner, than use some very smooth sand paper and get the runners where the manifold gaskets sit, cleaned.
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