This paper reports on a transient, three-dimensional computational fluid dynamics (CFD) study of flow and heat transfer in the complete fuel system of an inline 6-cylinder, direct injection (DI) diesel engine used in commercial applications. The CFD software Simerics-MP+ was used for this purpose. Diesel engine development, to meet fuel economy and exhaust emission standards, requires the precise integration of each component in the fuel system in order to reliably deliver the fuel to the combustion chamber as a function of crank angle to the combustion chamber, at the specified injection pressure. Both the model set-up and run times are practical, thus the simulation tool can play a key role in the design and development of diesel engine fuel systems.