An attempt to operate a diesel engine using the so-called hydraulic fuel injector with large clearance between the needle and its barrel was carried out experimentally. In this type of injectors, the spring force was substituted by a pressurized hydraulic oil to control the needle motion. Various factors affecting the injection system, for both hydraulically and spring loaded injectors, were also theoretically simulated and tested.The present study confirmed. according to both the experimental and theoretical results, that the use of hydraulic closer of needles is a promising technique to overcome the problem of diesel engine operation with large needle clearance (7 times the standard). To maintain the best specific fuel consumption by the engine, the injection timing has to be readjusted in accordance with the increase in the needle clearance. For example, for pintle nozzle (PIII6-2x25), as the clearance increased from 3 to 23μm the injection timing had to be retarded from 24° to 16° B.T.D.C. In order to maintain the optimum fuel consumption by the engine.