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High Idle After Starting

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My truck has the 5.3 AFM engine. After I start it " cold or warm " the idle spikes  to a little over 2000 RPMs and falls quickly to normal idle. This started last summer even in hot weather. If I drive 20 or more miles and shut it off, then immediately restart it, the RPMs quickly rise then fall to normal range within a few seconds. The computer shows no codes and the truck drives and behaves normally after that.

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