The Leathernecks (2-2) scored back-to-back TDs - the second after a Penguins fumble - to lead 45-24 in the middle of the fourth quarter.
The Penguins (1-3) scored on their next possession and recovered the ensuing onside kick, but Justin Fitzpatrick intercepted Montgomery VanGorder's pass in the end zone with 2:37 left to keep the lead at 45-31.
Youngstown State forced a three-and-out and needed only 1:03 for a nine-play, 66-yard drive that cut the deficit to a touchdown with 36.7 seconds left. Western Illinois secured the second onside kick and went into victory formation.
The Leathernecks led 30-10 on McGuire's 3-yard TD pass to Khalen Saunders with 5:38 left in the third quarter. The Penguins scored on a fourth-and-goal from the 2 early in the fourth quarter to cut the deficit to 30-24.
VanGorder threw for 418 yards, three touchdowns and two interceptions for Youngstown State.
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