After-injection is the introduction of additional fuel to the combustion chamber after the end of the main injection. It is a persistent diesel fuel injection problem which usually results in reduced engine power and economy and increased emissions. After-injection is caused by uncontrolled pressure transients at the injector after the opening of the pump spill port. These pressure transients are related to the wave propagation phenomena in the high-pressure pipeline connecting the pump and injector. Use of experimental trial-and-error methods in attempts to control this phenomenon has met with limited success.The analytical control method described in another paper is used to determine design means by which after-injection may be controlled. Further investigation and evaluation of two design changes which release the injection system excess elastic energy in a controlled manner are considered herein. One design change is the addition of a control valve in the pump delivery chamber. The other is the modification of the pump spill port. In both cases, pressures and flows are not altered during the main injection period. The ability of both design changes to control after-injection is confirmed by use of a simulation program. Experimental data from a system with the pump spill port modified in accordance with theoretical design calculations provide satisfactory confirmation of the analyses.