Chad Marshall had erased a 1-0 deficit in the 80th minute for the Crew on a set piece by Federico Higuain, who fed a pass that Marshall finished with a header over Montreal goalkeeper Troy Perkins.
Higuain -- who now has three goals and six assists in five games -- also set up Renteria's game-winner in the 93rd minute, delivering a pass to the Venezuelan for an easy shot past Perkins.
The Crew (12-8-6) used the victory to forge a third-place tie with Houston.
Marco Di Vaio scored in the 73rd minute for the Impact (12-14-3), who saw their five-game winning streak snapped.
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