We have carried out controlled-stress experiments in addition to the conventional controlled-rate measurements to probe the nature of nonlinear flow behavior of entangled 1,4-polybutadiene solutions. The flow responses are found to be drastically different depending on whether the shear flow is imposed by applying a constant torque or a constant velocity on one of the two surfaces in a cone-plate flow cell. When the applied shear stress is of a comparable magnitude to the elastic plateau modulus of the entangled solutions, a sharp yieldlike constitutive transition is observed, revealing a discontinuous relationship between the shear rate and the shear stress. Following such an entanglement-disentanglement transition (EDT), the chain orientation appears to further increase as a function of time as evidenced by the rising normal stress N-1, reflecting a plausible coil-uncoil transition (C-UCT). The relaxation of N-1 consists of an initial rapid decay, likely due to chain recoil from the C-UCT and a subsequent slow decrease characteristic of its relaxation in the presence of chain entanglement below the EDT. The controlled-rate measurements reveal familiar stress plateau behavior in a range of over 3 decades in the apparent shear rate, much of which is inaccessible by the controlled-stress experiment in steady state.