A variety of diesel fuels were investigated to develop an injector test to evaluate detergent activity. In this test a 6.2 liter GM engine with indirect injector capabilities was run for either 15 or 50 hr. The injectors were then rated by air flow for plugging. Our results indicated variability due to differences in fuels. A high mercaptan content fuel increased injector plugging. On the other hand, blends of base fuel with 30 or 50% Light Cycle Gas oil (LCGO) had no effect upon injector plugging even though total aromatics, including polyaromatics, and total sulfur contents, primarily thiophenes, increased. A low sulfur content California fuel decreased injector plugging. The results suggested that mercaptans increase injector deposits while thiophenes do not. Some work was also done using two commercially available detergents. Both were effective in reducing injector plugging at proper concentrations (keep clean). Cleanup capability was noted for one of these additives.