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An engine dynamometer test procedure was developed for evaluating fuel and fuel additive effects on the plugging of port fuel injectors. The test procedure was shown to adequately reflect the influence of fuels and additives on injector plugging in vehicles. Injector soak temperature and fuel system configuration were found to be critical factors in obtaining an acceptable engine-vehicle correlation. Injector plugging occurred in as little as 10 hours with a high-olefin base fuel; in contrast, plugging took two orders of magnitude longer with a high-quality, detergent-containing, commercial-type fuel. Furthermore, fuel additives greatly increased plugging resistance with the OEM, pintle-type injectors. Injector design alterations were also shown to be important. Pintle-type injectors with flared, aluminum caps extended plugging time considerably relative to the same injector design with plastic caps. Director plate-type injectors were much better still.