Q: Sporadic RPM surging while stopped in the traffic

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While I'm driving and am about to bring the car to a halt in the traffic, the RPM suddenly increases. When I stop, the RPM drops and then surges again. In other situations, the car idles at 700 RPM. This occurs sporadically. It doesn't matter if the engine is warmed up or cold. The dash isn't showing any lights on. I went to the dealership, and they had the wiring harness to the throttle replaced. The issue is still happening intermittently. The car has gone around 80,000 miles. Do you have any idea what might be wrong?

Fault codes should be read out of the ECM on your 2006 Mercury Montego. All connectors on the engine harness and at the ECU need to be checked for condensation and/or water ingress as these will cause drivability problems. I looked for relevant TSBs (technical service bulletins) and there are none published which cover the drivability concerns you have described. Being able to replicate the complaint while a scan tool is hooked up to look at sensor input settings will be imperative to solving this issue. If you’d like to have this checked, consider YourMechanic, a s certified technician will be able to come to your home or office to diagnose your rough idling issue and make the necessary repairs.

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