Damn, you guys made me drag out all my notes and books from my classes I took
When a car is started, the computer is in open loop until certain criteria is met. The o2 sensors need to be heated to work properly, so this is the main reason there is a delay.
After the sensors are heated up, the car goes into closed loop and the O2 sensors make their corrections.
The computer will go back into open loop under a variety of conditions. Throttle Position (TP) is one way.
There are parameters in the computer that determine what the TP is for Open Loop, and other things.
When that TP is reached, and the car goes open loop, and richens up.
In closed loop fueling, the computer takes readings from the oxygen sensors which read +/- of 14.64 AFR (Stoich) and makes corrections for it. If your car is idling @ 13.5 AFR, the 02s can see this, and will know to pull out enough fuel to get back to stoich.
In open loop, the fueling is purely based off the MAF transfer function and the open loop fuel table. This table has to be calibrated any time you make MAF changes, and changes that affect the airflow of the MAF.
With that being said.....as long as the 02 sensors are warm, he's in the loop..given the fact they are heated, it's likely the car is in fact in closed loop.