The 5.9L Cummins 12-valve diesel engines, manufactured between 1988 and 1998, feature a steel dowel pin that locates the front timing gearcase. Over time, this steel dowel pin can wobble out due to the constant heat cycles and vibrations of the engine. If (or when) the dowel pin works its way completely out, it falls into the timing gearcase.
When the dowel pin falls, one of several things can happen. The dowel pin can: