<div class="section abstract"><div class="htmlview paragraph">Exhaust gas recirculation (EGR) cooler fouling has become a significant issue for compliance with NO_x emissions standards. Exhaust gas laden with particulate matter flows through the EGR cooler which causes deposits to form through thermophoresis and condensation. The low thermal conductivity of the resulting deposit reduces the effectiveness of the EGR system. In order to better understand this phenomenon, industry-provided coolers were characterized using neutron tomography. Neutrons are strongly attenuated by hydrogen but only weakly by metals which allows for non-destructive imaging of the deposit through the metal heat exchanger. Multiple 2-D projections of cooler sections were acquired by rotating the sample around the axis of symmetry with the spatial resolution of each image equal to ∼70 μm. A 3-D tomographic set was then reconstructed, from which slices through the cooler sections were extracted across different planes. High concentrations of hydrocarbon is necessary for imaging deposits and only those coolers which exhibited large organic fractions or hydrated sulfate phases were successfully characterized. Cooler plugging and the effect of internal cooler geometry on the deposit thickness and spallation were characterized. Results are discussed in relation to future performance gains expected to occur with new spallation neutron sources that will provide energy selectivity and higher spatial resolutions.</div></div>