Q: Identifying cause of serpentine belt being thrown

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I have a 2007 Ford Fusion, 4 cylinder. The serpentine belt basically exploded, and I assumed that it was the result of wear; however, after replacing the belt and driving for approximately 100 miles, the new belt was thrown off. I have performed a spot check on each of the pulleys in the drive system, and all seem to be performing properly with the exception of the idler pulley, which wiggles very slightly with a small amount of pressure. I want to make sure I have identified the true cause of the belt being rejected before I replace any the pulleys. My question is, would the small amount of "play" or wiggle in the idler pulley disrupt the belt movement enough to throw it off of the drive?

My car has 180000 miles.
My car has an automatic transmission.

Hi there. It is always a good idea to replace all pulley’s when you have a catastrophic serpentine belt "issue" as you’ve described. In most cases, the force of the belt breaking will cause a slight warp of one of the pulley’s, which can cause the belt to become loose and break again quickly.

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