Fuel injector performance is critical for fuel efficiency, combustion process, emissions, start ability, acceleration and combustion noise. The injector design is a complicated process. Simulation tools are playing an important role in virtual design, which could evaluate performance and optimize the design. This paper describes how analysis is used to identify and resolve the cause of low kidney pressure when oil pressure in rail is high in a diesel injector. 1D system performance analysis tool and 3D CFD analysis tool are utilized together to identify the potential causes of the problem. The test results are compared with the simulation results to determine the root cause. 1D and CFD tools are used again to setup the design target and optimize the design. The test shows that the optimum design provided by 1D analysis and 3D analysis effectively solves the low kidney pressure problem in the fuel injector. The paper presents the analysis procedures and methods of root cause identification, design target setup, and optimization of injector design. It shows an example of 1D system simulation and 3D CFD simulation could work well together to solve the design problem and optimize the design.