Underhood/Underbody Thermal Management (UTM) is a key task during vehicle integration and validation process. It is enables reliability and durability of the automobile components. Furthermore, introduction of new non-metallic materials calls for a stronger demand of an accurate prediction of vehicle thermal signature under a wide spectrum of conditions. Computational Fluid Dynamics (CFD) has traditionally proven to give accurate predictions of full-vehicle thermal characteristics under a set of steady-state thermal inputs, representing the vehicle operating under a specific constant working condition (speed, RPM, etc). In reality, the thermal environment a vehicle goes through during operation is inherently transient and can change significantly within a very short period of time. This limits the usefulness of the steady-state simulations. Lack of temperature predictive capability during highly transient events posts risk of component failure due to unexpected thermal impact. The UTM CAE community shares the strong desire of performing transient thermal analysis for higher-fidelity simulation. However, running full transient thermal CFD analysis in a traditional fashion often yields to prohibitive computational resource. This works showcases a recent collaboration between General Motors and ANSYS on the investigation of a new simulation process to overcome high computational cost of transient UTM analysis. This is done by leveraging new functionality offered by ANSYS Fluent and creative engineering judgements and assumptions. As a result, this new process has revealed capability of capturing critical transient thermal responses for components of interest in a full-vehicle models. The analysis turn-around time is very reasonable using computational resources typically available to UTM CAE analysts today. In this work, details of this novel simulation process will be discussed. An example of the transient thermal behavior of a vehicle going through 20-minute highway cruise, 10- minute hill climbing, then coming to full stop for another 20 minutes will be showcased.