Misfiring or partial combustion during diesel engine operation results in the production of partial oxidation products such as ethylene (C₂H₄), carbon monoxide and aldehydes, in particular formaldehyde (HCHO). These compounds remain in the cylinder as residual gases to participate in the following engine cycle. Carbon monoxide and formaldehyde have been shown to exhibit a dual nature, retarding ignition in one temperature regime, yet decreasing ignition delay periods of hydrocarbon mixtures as temperatures exceed 1000°K. Largely unknown is the synergistic effects of such species. In this work, varying amounts of C₂H₄ and HCHO are added to the intake air of a naturally aspirated optical diesel engine and their combined effect on autoignition and subsequent combustion is examined. To observe the effect of these dopants on the low-temperature heat release (LTHR), ultraviolet chemiluminescent images are recorded using intensified CCD cameras. High-speed, crank-angle-resolved visible range images are recorded to examine soot-forming tendencies in the latter stages of combustion. Chemical kinetics modeling using a detailed hydrocarbon mechanism and a JP-8 surrogate blend is applied to track the behavior of HCHO, C₂H₄, OH, pyrene and HCHO*-producing reactions throughout the autoignition process. HCHO, in concentrations that may realistically be found to result from a partially firing or misfired engine cycle, is shown to effectively retard LTHR, increase ignition delay, and decrease the peak magnitude of the ARHR in a diesel engine operated with JP-8 under light load conditions. Further, this compound is found to suppress HCHO* chemiluminescence and act as an effective soot-promoter. Ethylene, C₂H₄, alone was not observed to influence LTHR or ignition delay, however, did marginally increase observed visible soot emission. Results identify formaldehyde as a species whose role and kinetic effects should be considered during diesel engine starting.