Q: My vehicle is idling at 1,500 rpms... why does this happen and should I be concerned?

High idle should be corrected as it will cause harsh transmission engagement, waste fuel, and make it difficult to stop the car as the engine surges ahead. High idle can be caused by a faulty idle air control valve (IAC valve). The IAC valve can fail or a faulty or blown fuse in other electronically controlled systems can cause the IAC motor to malfunction, leading to a higher than normal engine idle speed. Failures in the IAC valve itself are sometimes due to excessive carbon buildup in the intake manifold. Other possible causes of a high idle include vacuum leaks, EGR system malfunctions, malfunctioning oxygen sensors, dirty fuel injectors, fuel pump malfunction, faulty spark plugs, a faulty mass airflow sensor, a malfunctioning throttle and, rarely, a PCM/ECM malfunction. With regard to vacuum leaks, on a fuel injected engine a vacuum leak will cause a high engine idle as the engine’s oxygen sensor detect the extra oxygen entering the engine through the vacuum leak. The newly lean (too much air relative to fuel) status will result in the engine’s computer pumping in additional fuel, resulting in an abnormally high idle. A malfunctioning throttle system can cause both a high idle as well as an engine stall. A dirty air intake or cracked air intake tube is often the culprit. If you desire that this high idle problem be diagnosed by a certified Mechanic, dispatched by YourMechanic right to your location, please request an engine idle speed diagnostic - high idle and the responding certified mechanic will get this taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.