You could have a blockage in the intake tract or exhaust tract. One common thing is for catalytic converters to rust out and collapse in on themselves and block the exhaust. You can try banging on the converter with your hand or a rubber mallet to listne for losse metal.
Another problem might be the Idle Air Control Valve, IACV, which controls idle. It is located on the intake manifold near the throttle body. you can test it by removing it and leaving electrical connnector on, some have coolant lines running through them leave those one, and get an assistant to help you. Watch the solenoid plunger while your helper starts the car. It should move when the car starts.
Also look for carbon deposits in the trottle body by taking of intake tube. you may need to clean the thrttle body and IACV. When you do so use always spray towards the intake meaning the direction air would flow into the intake tract. Otherwise you could force depsoits deepr into the ports in the IACV.

Answered by DrCranknWrench (expert)
3,292 answers provided