This paper presents an analytical study of intake manifold fuel film transient behaviour. It is shown that the overall film response after a step change in airflow consists of two disturbances. A transverse wave imposes the new shear stress distribution, starting at the film surface and moving down through the film towards the wall normal to the film flow direction. A longitudinal wave advances down the length of the manifold parallel to film flow direction.The transverse wave is much faster than the longitudinal wave, is independent of air and fuel velocities and penetrates the film at a rate dependent only upon kinematic viscosity. The longitudinal wave depends upon new and old steady state conditions as well as fluid properties, and usually dominates the overall film response because of its much slower reaction. The model is used to examine transient engine behaviour, particularly under cold conditions.