Narrative:

The aircraft was dispatched with the aft pressurization outflow valve auto controller failed. The aft outflow valve; per the maintenance release; should have been inoperative and deactivated. During climb; the cabin altitude and rate of climb closely followed the aircraft altitude and rate of climb. At approx. 9;000 feet; with the cabin about 8;400 feet; we advised ATC that we could not climb higher than 10;000 feet due to the cabin pressure problem. We leveled at 10;000 feet; and the cabin stopped climbing at about 9;400 feet. We contacted dispatch and maintenance control for advice. After roughly 10 minutes; we decided that our off course vectors and extended flight at 10;000 increased our fuel consumption would probably not allow us to continue to ZZZZ. Also; concurrently; maintenance control had us select 'manual' on the aft valve control and attempt to control the aft valve manually. In manual mode; we were able to cycle the aft valve towards closed. At this point; maintenance control suspected an illegal dispatch due to a probable non-compliance with the maintenance release. Due to the fuel question; and the legality issue; we (maintenance control; dispatch; and myself) decided to divert back to ZZZ. ATC was notified of our intent to return to ZZZ; fuel was jettisoned per the emergency check list (ecl); and we configured the airplane; FMC; and navigation for a return to ZZZ; flight attendants and passengers were briefed/notified; with a normal ILS and landing runway 25L accomplished. An emergency was not declared. No aircraft limit was exceeded. No emergency authority used. After landing; it was discovered that the outflow valve system was not configured. Diversion/air return.

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Original NASA ASRS Text

Title: A Captain reports that an MEL Maintenance (M) procedure had not been complied with when an aft pressurization outflow valve auto controller was deferred on their B777 and the aircraft released for dispatch. During climb; the cabin altitude and rate of climb closely followed the aircraft's altitude and rate of climb. The aircraft would not pressurize; requiring a return to their departure airport.

Narrative: The aircraft was dispatched with the aft pressurization outflow valve auto controller failed. The aft outflow valve; per the Maintenance Release; should have been inoperative and deactivated. During climb; the cabin altitude and rate of climb closely followed the aircraft altitude and rate of climb. At approx. 9;000 feet; with the cabin about 8;400 feet; we advised ATC that we could not climb higher than 10;000 feet due to the cabin pressure problem. We leveled at 10;000 feet; and the cabin stopped climbing at about 9;400 feet. We contacted Dispatch and Maintenance Control for advice. After roughly 10 minutes; we decided that our off course vectors and extended flight at 10;000 increased our fuel consumption would probably not allow us to continue to ZZZZ. Also; concurrently; Maintenance Control had us select 'Manual' on the aft valve control and attempt to control the aft valve manually. In manual mode; we were able to cycle the aft valve towards closed. At this point; Maintenance control suspected an illegal dispatch due to a probable non-compliance with the Maintenance Release. Due to the fuel question; and the legality issue; we (Maintenance Control; Dispatch; and myself) decided to divert back to ZZZ. ATC was notified of our intent to return to ZZZ; Fuel was jettisoned per the Emergency Check List (ECL); and we configured the airplane; FMC; and navigation for a return to ZZZ; Flight Attendants and passengers were briefed/notified; with a normal ILS and landing runway 25L accomplished. An emergency was not declared. No aircraft limit was exceeded. No emergency authority used. After landing; it was discovered that the outflow valve system was not configured. Diversion/Air Return.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.