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Q: RPMs idling low.

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When I start up the car, the RPMs drop down low. And when at a stop the RPMs just idle up and down. The car has over 108,000 miles on it, and I didn't replace the spark plugs at 105,000. When it stalled on me three months ago, the dealer replaced the battery, ECU, and the alternator. What can be the problem?

Problems like your describing sound to me like a vacuum leak, or the throttle body might just be due for a cleaning. Modern throttle bodies are completely computer controlled and if carbon deposits aren’t cleaned regularly it can affect drivability and cause the hunting idle like you are describing, this is the computer trying to find the right mixture but can’t due to blocked passages or stuck idle control valves. Have a certified mechanic take a look on a scan tool capable of reading live data and read the fuel trim values, if the computer is demanding lean at idle and back to normal during acceleration its more likely a vacuum leak. If nothing is found I would recommend a throttle body and intake cleaning service.

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