Active automotive engine vibration isolation is considered where both stationary and transient engine-induced excitations as well as plant non-linearity are considered. The adopted control strategy targets the dominating spectral components of the excitation and achieves narrow band vibration isolation using feedback of disturbance states estimates. Time-varying gain-scheduled observer design, including investigations of closed-loop characteristics, is based on a linear parameter varying (LPV) approximation of the considered non-linear engine and subframe suspension system. To generate this representation, an approach of dividing the non-linear system into its linear and non-linear components where the latter is represented using a parameter dependent non-linear function is proposed. Parameter dependent quadratic stability analysis has been made tractable using an affine closed-loop system representation. High vibration isolation performance is demonstrated using co-simulations incorporating a detailed non-linear plant model and measured engine excitations. This is also achieved for engine operating conditions corresponding to rapid car accelerations, whereas the system exhibits non-linear characteristics and the fundamental frequency of the harmonic disturbance undergoes rapid time variations. Parameter dependent closed-loop quadratic stability is being shown assuming plant linearity. Yet, in the non-linear plant case, stability is guaranteed but only for limited intervals of the parameters and their time derivatives.