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Q: When stopped at a light rpm will dip from 600 to 500 can then move back up

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Engine has not stalled , but when coming to a stop the rpm will drop car shakes a bit then it will level back out , rinse and repeat. Also is there a way on this vehicle to set the rpm to about 700 ?

My car has 90000 miles.
My car has an automatic transmission.

A: Although the idle speed can be adjusted, th...

Although the idle speed can be adjusted, the idle speed of the engine is optimized by the manufacturer for maximum performance and economy of operation. In practice, therefore, there is no value in making any adjustment. Low and/or unstable idle is usually found in this list of causes: vacuum leaks, faulty idle air control valves(https://www.yourmechanic.com/services/idle-control-valve-replacement), dirty fuel injectors, incorrectly installed or damaged spark plugs, stuck EGR valves, defective or clogged fuel pump or filter, failing electrical components such as the ignition control module, plug wires, and coils, defective airflow sensor, dirty or failing oxygen sensor and a myriad of other causes. If the Check Engine Light has not set, and/or there are no diagnostic trouble codes to read, a quick strategy is to use an engine scope to evaluate each cylinder at idle. Such will usually indicate quickly whether there is a fuel or ignition system problem and also whether the rough idle is a "global" (all cylinder) issue or limited to one or a couple cylinders, due to, for example a failed injector, a failed plug wire or coil. If you desire a diagnostic and repair of this issue, please feel free to contact YourMechanic and request a rough running diagnostic which can be done at your convenience on a mobile basis.

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