The XUD9 IDI 6-hour steady-state test was developed to evaluate the nozzle coking propensity of diesel fuels. After the CEC 1997 Round-Robin, it was concluded that this test could not discriminate between fuels with the statistical confidence required for CEC tests. Furthermore, it was noted that the test would have to be considerably improved if it was to have any chance of meeting the CEC statistical standards.In 1998, Texaco proposed a 10 hour cyclic test as a replacement for the 6 hour steady-state test. A Task-Force was formed to investigate whether this test was a suitable alternative. After a mini Round-Robin, using the 10 hour cyclic test method, the high potential of the test was confirmed. Dynamic fuel injection timing was identified as the single most important parameter affecting nozzle coking.Cost-effective equipment was selected for measuring dynamic fuel injection timing accurately. The fuel pump was modified so that its internal automatic injection timing mechanism was locked to give a fixed injection timing. Several significant changes were made to the test method to ensure better control of the test. Another Round-Robin was carried out by Task-Force members to evaluate the effect of all these changes to the test. The results demonstrated greater consistency of data and good discrimination between fuels.In total, after 8 Task-Force meetings and numerous tests it was concluded that the 10 hour test had significantly higher discrimination than the 6 hour test. In addition, the mini Round-Robin data analysis indicated that the test met CEC statistical requirements. The EFTC ratified the findings, which enabled the Task-Force to proceed with the development of this test and to conduct a full Round-Robin.