Ironically, I had this exact same failure on the takeoff roll last night (Eicas advisory alert OUTFLOW VALVE L at about 100 KIAS), albeit on a 747-400.
We engaged the A/P, actioned the QRH checklist, switched off one pack and drove the failed left outflow valve closed manually (so using DC motor). As far as I am aware the CPC automatically modulates outflow valve position using the AC motor, while DC motor is only for manual control; for sure that valve was going nowhere until we ran it closed manually.
From a practical perspective we noticed that the normal SOP of first cleaning up the airplane and then actioning the QRH might not be feasible: in our case the left valve had failed almost fully open, and even with the right one closed and three packs running the cabin was climbing at well over 1000 fpm, the old ears were starting to hurt. We elected to clean up partially and then action the checklist.