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The focus of this work is to understand how diesel engines “wetstack.” Mainly through the work which will be discussed here, the root cause appears to be the presence of polycyclic aromatic hydrocarbons (PAHs) in the exhaust and accumulation on the small clearance between the exhaust valve stem and the valve guide. Experimental data which support this premise was obtained by operating an engine at no load and collecting exhaust soot particulate on a filter paper onto which PAHs were adsorbed. Supercritical carbon dioxide extraction was then used extract PAHs for analysis. The extract was processed by GC/MS to quantify PAHs. Because of the high unburned fuel carry-over at these operating conditions, it was necessary to use tandem MS to isolate each PAH from fuel carry-over in the spectra.