Aftermarket dual-fuel injection systems using a variety of different fumigants have been proposed as alternatives to expensive after-treatment to control NOx emissions from legacy diesel engines. However, our previous work has shown that available add-on systems using hydrous ethanol as the fumigant achieve only minor benefits in emissions without recalibration of the diesel fuel injection strategy. This study experimentally re-evaluates a novel aftermarket dual-fuel port fuel injection (PFI) system used in our previous work, with the addition of higher flow injectors to increase the fumigant energy fraction (FEF), defined as the ratio of energy provided by the hydrous ethanol on a lower heating value (LHV) basis to overall fuel energy. Results of this study confirm our earlier findings that as FEF increases, NO emissions decrease, while NO2 and unburned ethanol emissions increase, leading to no change in overall NOx. Peak cylinder pressure and apparent rates of heat release are not strongly dependent on FEF, indicating that in-cylinder NO formation rates by the Zel'dovich mechanism remain the same. Through single zone modeling, we show the feasibility of in-cylinder NO conversion to NO2 aided by unburned ethanol. The modeling results indicate that NO to NO2 conversion occurs during the early expansion stroke where bulk gases have temperature in the range of 1150–1250 K. This work conclusively proves that aftermarket dual fuel systems for fixed calibration diesel engines cannot reduce NOx emissions without lowering peak temperature during diffusive combustion responsible for forming NO in the first place.