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I am just wondering about the general idea. So, when I do not have any gear put in and increase the revs to let's say 4000 rpm and then remove my foot from the gas pedal the revolutions go down rapidly then seem to stall for a tiny bit at certain revs (I think at ~1500 rpm) and then continue to drop to idle at a visibly different rate. I realize the engine slows down when no fuel is fed for obvious reasons, but clearly there is a pattern in this behavior.

In my car (Fiat Barchetta '95) there is a cable connecting gas pedal to the throttle and there is a throttle bypass via the idle control valve (ICV). I am assuming that when I remove my foot from the pedal the throttle closes down almost entirely (right?) and then there is this ICV that might let some air through and adjust engine rpm. I would really appreciate if someone could share his thoughts on this subject.

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You have it right on the nose.

When the car is idling there is fixed amount of air passing though the throttle plates and the rest of the air is controlled by the ICV or IAC (same difference). By controlling the air intake rate the idle is controller.

When the car goes off idle the throttle plates control engine speed. At this point the IAC goes full open or near full open. This is so that if the throttle is snapped shut the engine won't starve for air. By itself the IAC is not fast enough to compensate for that rapid of an air change.

When the throttle is snapped shut, because the IAC is fully open the car will idle high. Then when the computer has verified that it has control of the idle it will slowly close the IAC to reduce the idle to the desired rate.

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While I agree with vini_i, I have one other reason for this behavior: Fuel cut-off

When the gas pedal is released at higher RPM, the motor doesn't inject any fuel to save some of it, and if the car has a display, it shows 0.0l/100km. (No idea what it shows in mpg unit) Below a threshold of about 1500RPM, the motor starts to inject fuel again to prevent stall, and the car displays a consumption below 1.0l/100km.

You can even hear and feel it in some manual cars, when you let them roll in idle. When the RPM falls below ~1500RPM, you can hear and feel that the motor starts to burn fuel again, and the braking power of the motor is reduced.

Though, I'm not sure if fuel cut-off is activated within that 1-2 seconds the motor needs from 4000 to 1500RPM in your case.

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