Q: Why is my car idling in this weird way?!

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When car is in motion and fully warmed up, when clutch disengaged or in neutral, the RPM will drop but pulse between 300-800 continuously until car comes to a stop. Once stopped it will slowly return to 650 RPM (Normal RPM). Sometimes it won’t stop pulsing at all. Headlights will dim and brighten along with the pulsing. Just had alternator and battery replaced. Before that service was done, the idle was way worse but is still happening. No check engine light or codes as well. Car drives perfectly fine, no loss of power and no change in gas mileage. Please help!

My car has 123000 miles.
My car has a manual transmission.

Manual transmission equipped vehicles have the usual idle air control circuitry as well as an electronic type dashpot to modulate (slow down) the fall off in RPM when you take your foot off the accelerator. There is likely a fault in those latter systems and that fault is causing the phenomena you have described. There are other possibilities, too, though, including vacuum leaks, EGR system malfunctions, malfunctioning oxygen sensors, dirty fuel injectors, fuel pump malfunction, faulty spark plugs, a faulty mass airflow sensor, or a faulty throttle position sensor. The root cause can be pinpointed, and the required repairs made, if you request an engine diagnostic. If you have further questions or concerns, do not hesitate to re-contact YourMechanic because we want you to make the most of your repair dollars and help you to get the best possible results.

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