To meet increasingly stringent diesel exhaust emissions requirements, original equipment manufacturers (OEMs) have introduced common rail fuel injection systems that develop pressures of up to 2000 bar (30,000 psi). In addition, fuel delivery schemes have become more complicated, often involving multiple injections per cycle. Containing higher pressures and allowing for precise metering of fuel requires very tight tolerances within the injector. These changes have made injectors more sensitive to fuel particulate contamination.Recently, problems caused by internal diesel injector deposits have been widely reported. In this paper, the results of an investigation into the chemical nature and probable sources of these deposits are discussed. Using an array of techniques, internal deposits were analyzed from on a number of sticking injectors from the field and from OEM test stands in North America. In each case, the internal deposits were found to be composed mainly of the sodium salts of alkenyl succinic acids. These salts are insoluble in ultra-low sulfur diesel (ULSD) fuel and can exist as very fine particles that pass through fuel filters, flowing to the rail and injectors. Sodium can enter diesel fuel from a number of sources including refinery salt driers, storage tank water bottoms and seawater used as ship ballast. Alkenyl succinic acids are commonly used as corrosion inhibitors to protect pipelines and other parts of the diesel fuel delivery system. Based on the proposed mechanism, an engine test was developed that reproduces the deposits and injector sticking observed in the field and OEM test stands. This test was used to identify alternate corrosion inhibitor and detergent chemistries that significantly reduce the propensity to build sticking deposits in low clearance areas inside high pressure diesel injectors.