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is this a 5 speed car?
If so, then I bet it is "desired" by the ECU.
The ECU cannot tell what gear you are in on a 5 speed. And there are less pumping lossses with a higher throttle opening on deceleration with the car in gear. So I bet the ECU is opening the IAC valve to let the car coast "better", and you are fooling it by shifting into neutral.
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