Abstract: State-of-the-art fuel injectors can be defined by a third-order polynomial to define their performance curve and to optimize performance of the fuel injected engine. Such systems are preprogrammed to use a set of fixed coefficients when replacing fuel injectors. Therefore, these replacement fuel injectors must be manufactured with very precise tolerances so as to operate efficiently with these fixed coefficients. The present invention includes a method and apparatus that allows the use of production fuel injectors that are more economical since they can be manufactured with wider tolerances. The production fuel injectors are supplied with a computer program and a data file having a set of replacement coefficients that are determined especially for that particular fuel injector. The approach allows the restoration of the existing coefficients if the replacement fuel injector does not solve the service problem experienced by the user of the fuel injected engine.