Trailing 19-14 after a 37-yard Luke Paladino field goal, Valparaiso had driven to the Marist 11 with 20 seconds left to play when McElroy intercepted a Chris Duncan pass and returned it all the way to seal the win.
Cobb, with 92 yards on 18 carries, scored runs of 16 and 60 yards and the Red Foxes (4-7, 4-4) led 16-8 at halftime.
The Crusaders (1-11, 1-7) closed to 16-14 on Duncan's 9-yard TD pass to Ollie Reese, and got the ball back with 2:34 to play after Paladino's field goal.
Luke Strnad was 10 of 18 for 147 yards with an interception for Marist. Anthony Olivencia had three catches for 102 yards.
Duncan was 17 of 43 for 232 yards with two TDs and two picks for Valparaiso.
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