Been having sputtering problems, and found out my timing belt skipped a tooth, or two. Having it replaced and fixed this weekend but wanted to know what causes it. I’m at 110K and my belt & water pump were replaced at 75K.

 

I would guess the tensioner is weak or something else wrong with it. When I changed the belt on my Accent it was recommend to change the tensioner pulley if grease was leaking from the bearing, which there was.