A second successive scoreless stalemate means Alan Curbishley 's side has only been breached once in the last seven top-flight clashes on home territory.
After a tight and scoreless first half, the Reds broke the deadlock early in the second period.
City boss Dave Jones kept faith with the side held to a scoreless draw by Derby.
Following a scoreless first period, the home side took the lead midway through the second half through Marco Andreoni.
Neither team could break the deadlock and the game remained scoreless at half time.
Portugal had the ball in the net through sub Postiga but he was well offside and the game was destined to end scoreless.
Saturday 's scoreless tie with the lowly Columbus Crew, however, suggests the Fire better turn things around sooner rather than later.
The defense was smothering, and even after Wolves Ty Shaw went to the free throw line, they were still scoreless.
The match between Lille and Benfica has also finished scoreless, so United have their destiny in their own hands.