Q: My car idles rough when stopped

This may be a sign of a leaky injector or potentially an incorrect idle air adjustment. A leaky injector can cause a rich fuel condition as it may leak too much fuel into a cylinder not allowing the combustion cycle to burn the fuel evenly resulting in excess unburnt fuel being unused and momentarily flooding the motor causing this kind of rough idle. When this happens, you may notice a bit of black smoke coming from the exhaust as well as a slight fuel smell.

This may also be a slightly misadjusted throttle plate. The throttle plate is adjusted at a specific angle to allow just the right amount of air into the throttle body at idle speeds. This is done by the work of an actuator motor that electronically adjusts this. When this is not working properly, this may cause a rough idle due to an unbalanced load of air and fuel to be supplied to the motor.

To avoid any unnecessary repairs, I would recommend having an expert from YourMechanic come to your location to diagnose your rough idle firsthand.