This is usually caused when the IAC (idel air control) valve is not
functioning propery. The IAC is a silver cylinder about 1.5 inches in
diameter bolted to the side of the intake manifold. It is held on by
two screws with an electrical connector on one end. You can usually
restore it to operation by removing it and cleaning the rod you can see
throught the opening when you remove the IAC from the intake. I usually
just replace them ($50 or so from NAPA).
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