The contributions of vibration sources in a gasoline direct-injection (GDI) engine are identified by simulating a two input-one output model. A partial coherence function between two inputs and one output was calculated to identify the contributions of the two inputs to the output. In order to evaluate the proposed method, a principle test was performed using a laboratory test rig. The results show that the model can be used to identify the contribution of each input to the output. Finally, this model is applied to the identification of the vibration source in a GDI engine.