The unstart phenomena in a model scramjet with the freestream of both low and high enthalpy Mach 4.5 flow conditions at an arc-heated hypersonic wind tunnel are investigated. Then, the unstart phenomena induced by a nitrogen or ethylene jet at low and high enthalpy conditions are compared. The nitrogen or ethylene jet pressurize downstream by mass addition and heat release from combustion. High-speed schlieren at the jet and the lip of the scramjet model inlet and high resonance frequency surface pressure measurements are used to capture flow features during an unstart process. In both conditions, the similar transient behavior of unstart shockwave system spawned by the flow choking and quasi-steady state of the unstart shockwave system at localized favorable pressure gradient are observed. In combustion driven unstart process, severe oscillatory flow motions of the jet and the unstart shockwave at the lip of the model inlet are observed. On the other hand, the unstarted flow motions triggered by mass addition remains relatively steady after the completion of the unstart process. The discrepancies between the unstart processes induced by the nitrogen and the ethylene jet are explained by flow choking analysis.