The method comprises the steps consisting in assuming that the
gain of at least one
injector is equal to a theoretical
gain or an updated
gain, and in replacing each of at least one reference injection of injection control duration controlled by the
engine control unit in application of a stored characteristic, with a
multiple injection comprising a succession of at least two injections of injection control durations that are assumed to cause the same
mass of fuel to be injected as by the replaced reference injection, in determining the fuel
mass difference between the reference injection and the
multiple injection, in deducing therefrom an error in determining the characteristic, and in modifying the gain and / or the offset of the linear zone of the initial characteristic or at least a table or
mathematical relationship for the non-linear zone of said characteristic, so as to compensate the error, and storing the new characteristic as determined in this way. Applications to real time determination as a function of the injection control duration of the flow rate characteristic of fuel injectors for internal
combustion engines fed by injection.