The paper presents experimental and theoretical results obtained for a mechanical Diesel fuel injection system, made up of a distributor-type pump, four delivery pipes and four four-hole injectors. Pressure in the pumping chamber, in two locations along the fuel line and within the injector is measured directly, as well as the injector needle lift. The flow rate is evaluated through the measure of pressure in the injection chamber.Experimental results are sustained by theoretical results. The numerical model considers systems of ordinary differential equations representing the operation of injector, pump, delivery valve and line volume elements. Only a few model details are presented. Similar approaches are in use by many years, and the accuracy they provide is generally accepted to be fairly good. Theoretical and experimental results are presented vs. the time at different pump speeds, showing a very satisfactory accuracy.The main objective of the paper is to describe the unsteady flow within the injector, and to show differences with reference to the steady flow. Despite the subject is not adequately covered in the open literature, several reasons suggest that steady and unsteady flow behaviors differ considerably. These differences may affect the prediction of both injection pressure and flow rate.