Idle - RPM increase

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If you have a Manual Transmission, shift into neutral and the RPMs increase (i.e. from 700 to 1100) - what is the likely cause?
Vehicle motion causing more air entry and therefore raising the required RPM or...
The car moving giving a signal that it needs to maintain a higher idle for some reason?

The testing I've done suggest that as the car slows, the RPMs drop back down - but above say 40 or so, the RPMs rise as if there is some reason to maintain a higher RPM.

[arcosine]

Car is programed to do this from the factory. You have to plug the auxillary air intake port in the throttle body.

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Why does it want to do that - i'm looking but yeh, throttle body auxillary port isn't helping. If I block it and am idling won't that be affected (throttle blade shut, so no air?).

[Awd180]

Clean the IACV and MAF, you more likely idle at 800-900rpm normally, some cars do idle lower. Are you reading a digital or analog gauge? Messing with vacuum lines on the throttle body isn't a good suggestion, I deleted the purge control system on my 96 saab and it hasn't really changed anything. Deleting the IACV will cause a rough idle leading to more issues, staling and poor throttle response.

[Occasionally6]

It is usual for the ECU to be programmed so that when the car is away from idle, the idle air control valve is opened. It may be you are just picking that up. There can be several reasons for the IACV opening.

One is that it allows what is called "sail on" - the car will maintain speed when the throttle is lifted, and not immediately decelerate rapidly due to engine drag. (My car will do a remarkably good job of maintaining a given road speed down hill on a closed throttle no matter what the incline.)

Another is that it can be used to alter the relationship between accelerator pedal position and the air intake into the engine. That can be used to mask driveability problems and more accurately match the pedal position to a torque demand. It's the poor man's DBW throttle.

Even with modern engine management there will be a HC spike with a rapid throttle shut off, so the IACV is used to damp the change in airflow and reduce the associated exhaust emissions.

It can be used to effectively increase the size of the throttle body and so increase wide open throttle air flow capacity (maximum power).