An experimental study of the 1st order vibration improvement at engine idle condition of a malfunctional vehicle in development is presented in this work. Firstly, the generation mechanism of the vehicle 1st order vibration problem at engine idle is analyzed based on theoretical analysis. Then, experiments are designed to derive the possible vibration excitation source and transfer path through comprehensive analysis. The powertrain rotational system imbalance exceeding a limit is determined to be the main reason for the vehicle 1st order abnormal vibration. The resonance of the mounting system excited by the powertrain rotational system intensifies the 1st order abnormal vibration. Experimental analyses show that the clutch imbalance severely exceeds the prescribed limit, which confirms the diagnosis of the excitation source, and a significant improvement of about 87% of the 1st order vibration can be achieved by adjusting the clutch imbalance.