The Port Fuel Injector (PFI) Deposit Test is a performance-based test procedure developed by the Coordinating Research Council and adopted by state and federal regulatory agencies for fuel qualification in the United States.To date, Southwest Research Institute (SwRI) has performed over 375 PFI tests between 1991 and 1998 for various clients. This paper details the analyses of these tests. Of the 375 tests, 199 were performed as keep-clean tests and 176 were performed as clean-up tests. The following areas of interest are discussed in this paper: Keep-clean versus clean-up test procedures Linearity of deposit formation Injector position effects as related to fouling Dirtyup / cleanup phenomena Seasonal effects This paper draws the conclusion that it is easier to keep new injectors from forming deposits than it is to clean up previously formed deposits. It was found that injector deposit formation is generally non-linear. When keep-clean tests fail, they usually fail less than 8,045 km with injectors two and three the usual positions that fail. During the dirty-up phase, injectors two and three reach 10% fouling more often than injector positions one and four. The typical clean-up patterns that are encountered consist of either fast dirtyup / fast cleanup or slow dirtyup / slow cleanup. Pass / fail test results for the clean-up tests are dependent on the time of year. However, there is not a significant difference in the pass / fail test results for the keep-clean tests between seasons. Differences in the fouling tendencies of fuel injectors are not completely understood.