Q: My car rough idles and stalls, but no check engine light.

This issue is normally caused by low fuel pressure or a faulty injector, but since you have just replaced the fuel pump, this may also be a sign of a failing idle air control (IAC) valve. This is a unit that is located on or near your intake manifold that monitors the air intake as it is mixed with fuel prior to being injected into the engine. This valve is controlled by the vehicle’s computer which will adjust idle speed based upon other measurements such as engine temperature, intake air temperature and electrical system load or voltage. When you accelerate, the engine RPM increases, and as you let off the gas, the RPM slowly returns to the normal idling speed. However, when the engine RPM drops below the normal range of 800 RPM (for most cars), it is more than likely that you are dealing with an IAC valve problem. In cases where this valve is clogged or is nearing its replacement age, your vehicle will stall instead of idling after the release of the throttle. If you’d like assistance with this issue, a certified YourMechanic technician can come to your location to help inspect your vehicle’s rough idle or replace the IAC valve for you.