How does ECU decide how much air is needed during idle?

[freeonboard]

re stock 92 s13

In theory, ecu ignores o2 sensor readings so how does it decide on a good idle. My ACC is definitely working but it insists on being 50 percent open during idle which shoots my idle to 1000 rpm. If I unplug the acc, the idle is around 700 but the ecu cries for air by sending a message to acc to open up 80 percent. The ACC goes down to about 30 percent when I go into open loop. It stays at 50 percent open even when I go into timing mode. I have 4 computers and they all do this. I'm thinking I'm rich during idle but curious how the ecu determines this.

[480sx]

The main IACV works on coolant temp. Its really freaken simple, it gets a 12 volt signal when the cars on, and works. The IAC-FCID valve is a bish, thats the one thats AC activated, then its PWM, ect.

[freeonboard]

The main IACV works on coolant temp. Its really freaken simple, it gets a 12 volt signal when the cars on, and works. The IAC-FCID valve is a bish, thats the one thats AC activated, then its PWM, ect.

thanks for the response

the ka24de 240sx doesnt have coolant flowing through iacv. The ecu decides on percentage and that is pretty much fixed during idle. It starts off at 50 percent when engine is cold and does not change when engine is warmed up. When you first start, it is open about 70 percent momentarily.Bumps up when you use power steering etc. also. The air regulator supplies the extra air when engine is cold.

[480sx]

FSM is your friend!

Heh. I have taken apart more Ka's than well..

Coolant flows directly to the IACV. Its a heat activated valve that controls the amount of air that bypasses the throttle body depending on coolant temp.

If you'v noticed, the IACV has a single, two wire plug connected to it. On/off. Simple as that.