Many dual fuel technologies have been proposed for diesel engines. Implementing dual fuel modes can lead to emissions reductions or increased efficiency through using partially premixed combustion and fuel reactivity control. All dual fuel systems have the practical disadvantage that a secondary fuel storage and delivery system must be included. Reforming the primary diesel to a less reactive vaporized fuel on-board has potential to overcome this key disadvantage. Most previous research regarding on-board fuel reforming has been focused on producing significant quantities of hydrogen. However, only partially reforming the primary fuel is sufficient to vaporize and create a less volatile fuel that can be fumigated into an engine intake. At lower conversion efficiency and higher equivalence ratio, reforming reactors retain higher percentage of the inlet fuel’s heating value thus allowing for greater overall engine system efficiency. The experimental work described in this paper successfully demonstrates that a thermally integrated reforming reactor using exhaust gas recirculation mixed with vaporized diesel fuel as reactants can effectively reform diesel fuel with high conversion efficiency. The engine was operated at two speed and load settings and achieved lower soot, but higher CO and THC emissions. Overall engine efficiency was decreased compared to conventional diesel combustion; loss of fuel heating value in the reforming reactor did not allow the engine to achieve efficiencies found for operation with no reforming. Additional work is required to improve diesel fuel vaporization prior to the reactor and to operate at higher reformer equivalence ratio, thereby increasing overall engine thermal efficiency.