A belt tensioner for a power transmission belt may be provided that operates on an endless path and that utilizes asymmetric motion control. The belt tensioner may have an arm with a belt engaging section and a drum section, a support member for securing the tensioner relative to the belt, where the arm pivots on the support member, and a tension spring that urges the arm to pivot about the support member in a first direction and urges the belt engaging section against the belt with a force to tension the belt. The tensioner also may have a fluid filled chamber inside a portion of the drum section of the arm and a valve pivotally attached to the tensioner so that the valve extends across the fluid containing chamber.