timing belt crank pulley on 1998 Isuzu Oasis

the timing belt keeps slipping off which will not allow the van to start it will turn over but will not keep running. while tryinfg to take parts off the crank pulley crumbled i can get oneaat the junk yard but what is the problem with the timing belt slipping off What d I do about this

Asked by for the 1998 Isuzu Oasis
Most timing belt systems have a least one timing belt tensioner go to but in your vehicles info and put belt in the search box and look at the diagram
My Isuzu van year1996 timing belt is new but it slip off the pulley if pulley ring is loose what do I do