A vehicle has a vehicle system with a waste heat fluid. An expander, a condenser, a pump, and an evaporator are provided in sequential fluid communication in a thermodynamic cycle containing a working fluid. The evaporator is configured to transfer heat from the waste heat fluid to the working fluid. At least one valve adjacent to the pump is controlled to control fluid flow through at least one chamber to maintain a pressure of the fluid at a pump inlet at a threshold pressure above a saturated vapor pressure associated with a temperature at a condenser outlet when ambient temperature varies.