Foul taste in mouth because of expander?

Food stuck on it. Be sure to rinse vigorously after eating to help dislodge any food that may be stuck underneath. Also, be sure to use a water-pik or a syringe to squirt water/mouthwash under it before you brush. After brushing, rinse vigorously with an antibacterial mouthwash.
See a dentist. Brush, floss, and use a listerine-type mouthwash. If the taste/odor doesn't go away, see your doctor to rule out a throat infection, then see a dentist to evaluate for a tooth abscess.