Closed loop control is maintained at idle on all MBs that have closed loop control. Most go open loop on full throttle.
The reading at the diagnostic conn is a created reading often called integrator. It is a duty cycle and the point of view of a duty cycle is critical. Fifteen percent duty cycle is also eighty-five percent duty cycle. If you look at what is happening it is easy to understand. You might want to look at the scope patterns in this article:
http://www.peachparts.com/Wikka/EngineControls
The figure one that shows 30% duty cycle is refering to the amount of time the voltage is at 0.0v versus the amount of time it is at battery voltage. This is 30% off duty cycle. It is also 70% on duty cycle.