Exhaust gas recirculation (EGR) cooler fouling has become a significant issue for compliance with NOX emissions standards and has negative impacts on cooler sizing and engine performance. In order to improve our knowledge of cooler fouling as a function of engine operating parameters and to predict and enhance performance, 19 tube-in-shell EGR coolers were fouled using a 5-factor, 3-level design of experiments with the following variables: (1) EGR flow rate, (2) EGR inlet gas temperature, (3) coolant temperature, (4) soot level, and (5) hydrocarbon concentration. A 9-liter engine and ULSD fuel were used to form the cooler deposits. Coolers were run until the effectiveness stabilized, and then were cooled down to room temperature and run for an additional few hours in order to measure the change in effectiveness due to shut down. The coolers were cut open and the mass per unit area of the deposit was measured as a function of distance down the tube. Microstructural analysis revealed that the deposit mass and thickness were generally lowest both at the cooler outlet and on the upstream side of the turbulators for all the coolers. A low EGR inlet gas temperature and high EGR flow rate produced the thickest deposit and the highest loss of effectiveness. High EGR inlet gas temperatures appeared to produce mud-cracking in the deposits and when combined with high EGR flow rate, often resulted in deposit spallation near the inlet. Coolant temperature, smoke level and HC concentration had less of an effect on cooler performance.