I just came from newer Mustang, 2008, same, no mixture change, OL or CL, there is fixed wideband installed. I tested only idle this time.
edit, additional test.
20 percent rich at idle, 0 part throttle and 10 lean wot, no matter what LWFM set, even zeroed, maxed. Car drives normal.
As on another tested before...
So again, if someone will have 3v in his hands, please do same thing for test and let here know how it looks.