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Q: Truck idles poorly, starts jumping

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Lately, the truck has developed some idling problems. It occurs when I'm driving and stop at a traffic sign for instance. Then while it's idling, the truck begins to jump. It would return to normal if I put it in neutral but it only lasts for a short while. It would then start jumping again until I accelerate. What can cause such poor idling?

A: There are numerous potential causes for the...

There are numerous potential causes for the symptoms you are having. The first thing that comes to mind is a vacuum leak (unmetered air allowing the engine). Vacuum leaks are noticeable at idle and not under load while accelerating. There are several DIY methods for finding vacuum leaks, but the best method is to use a professional smoke machine. Another potential cause for your issue is a faulty idle air control valve (IAC). The IAC is a stepper motor, used by the engine control module (ECM) to control throttle position. The ECM uses the IAC to control idle speed and prevent stalling. If the IAC is not working properly, the vehicle will not idle well. I suggest you have a trained professional, such as one from YourMechanic, inspect your vehicle firsthand to diagnose your rough idle.

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