This article is a continuation of an earlier article with the same title which extended the analysis of the current-voltage curve of the flame ionization detector near the origin of the curve, in terms of both mobility and diffusion of the flame ions oxonium hydrates. It is shown here that diffusion dominates the current only when the value of the applied voltage is much less than 1 V. The theory of the instrument using only diffusion is given in one, two and three dimensions. Experiments have been performed on a flame ionization detector using applied voltages up to 5 V and the current extrapolated to zero applied voltage. From this value of the current the ionic diffusion coefficient was obtained which agrees with that deduced from the Einstein relation and the measured value of the mobility. The theory also yields a method of obtaining the diffusion coefficient from the slope of the current-voltage curve at the origin but the coefficient so obtained is incorrect because the voltage interval of 1 V is too large and to use the theory successfully would need the region of the current-voltage curve much closer to the origin.