Q: I had a new serpentine belt and pulleys put on my car but the belt tends to keep popping off,

The serpentine belt need constant tension on the belt to keep it from coming off. If the belt alignment is not aligned then the belt may come off. The tensioner may be out of alignment and not putting enough tension on the belt. Have a mechanic check the tensioner and replace it if needed.

Hello, It sounds like you may have a loose idler pulley or a tensioner pulley that may be out of adjustment. The tensioner pulley is a pulley in the serpentine belt pattern that is used as an adjustment pulley to increase or decrease tension put on the belt as needed. When this is out of adjustment or the tensioner pulley itself is loose this may cause the belt to come off as you describe. I would suggest having a professional from YourMechanic come to your location to diagnose and inspect your vehicle.