Can someone explain the difference between closed and open-loop mode on an ECU?

 

closed loop= use of 02 sensor
02 sensor will make adjustments.

Open loop= not using 02 sensor
Whatever your maps dictates.

 

and the ecu alternates between the two according to throttle postition, load, and engine temp to get the best fuel mileage and driveability. It might look at more sensors than that to decide but I'm sure it looks at TPS, MAP, and coolant temp.

 

The way I understand things, closed loop is mostly for partial throttle cruising, and open loop mode is always used when you are wide open.