Fundamental understanding of the sources of fuel-derived Unburned Hydrocarbon (UHC) emissions in heavy duty diesel engines is a key piece of knowledge that impacts engine combustion system development. Current emissions regulations for hydrocarbons can be difficult to meet in-cylinder and thus after treatment technologies such as oxidation catalysts are typically used, which can be costly. In this work, Computational Fluid Dynamics (CFD) simulations are combined with engine experiments in an effort to build an understanding of hydrocarbon sources. In the experiments, the combustion system design was varied through injector style, injector rate shape, combustion chamber geometry, and calibration, to study the impact on UHC emissions from mixing-controlled diesel combustion. A tracer-based visualization methodology, using full cylinder and full cycle CFD simulations was developed to understand combustion chamber exhausting processes and help interpret crank-angle resolved test measurements of UHC in the exhaust runner using a Cambustion Fast Flame Ionization Detector. Comparisons of the quantitative CFD predictions of UHC for varying engine configurations gives additional insight on the current state of the art predictive capability. Simulations with imposed fuel injector dribble phenomenon were additionally evaluated. The historical view of diesel UHC emissions having a strong dependency on injector dribble and injector sac volume was confirmed to still be of utmost importance. Results indicate that fuel injector dribble is a key source contributing from 75 to 90% of the UHC emissions under the conditions investigated. A discussion of simulation gaps and future modeling needs specific to diesel hydrocarbon sources highlights CFD capabilities required for research and development environments.