Open loop fueling is used in cars equipped with narrow band O2 sensors to determine the injector on time w/o using feedback from the sensors. Generally, it is based on the amount of air metered by the mass air flow sensor, and whether the desired AFR is stoichiometric (lambda=1) or not.
Motronic will go open loop under any of the following conditions
- At motor start
- When load > RLLRUN + TRAB delay time
- When req lambda != 1
Note that there are other open loop conditions, such as during idle under certain circumstances, and at fuel-cutoff during overrun.