Abstract: This invention pertains to an inline pump coupling system for use with a diesel engine, designed to place an additional high pressure oil pump behind the existing high pressure oil pump for the diesel engine, and which doubles the high pressure oil volume output from the pumps to the diesel engine injectors. The pump coupling system consists of a replacement shaft portion, the pump coupler that fits intermediate the pair of aligned pumps, and a shaft coupler portion, for coupling the shafts of the inline pumps together. The replacement shaft has a shaped back end that matingly engages within the shaft coupler of the back pump, to provide for simultaneous rotation of the pump shafts during generation of the high hydraulic oil pressure developed from a simultaneous operation of the pair of inline pumps.