Improved submodels for use in a dynamic engine/vehicle model have been developed and the resulting code has been used to analyze the tip-in, tip-out behavior of a computer-controlled port fuel injected SI engine. This code consists of four submodels. The intake simulation submodel is similar to prior intake models, but some refinements have been made to the fuel flow model to more properly simulate a timed port injection system, and it is believed that these refinements may be of general interest. A general purpose engine simulation code has been used as a subroutine for the cycle simulation submodel. A conventional vehicle simulation submodel is also included in the model formulation. Perhaps most importantly, a submodel has been developed that explicitly simulates the response of the on-board computer (ECM) control system. Within this ECM submodel, calculations are performed to determine the requested equivalence ratio, the delivered equivalence ratio, the duration of combustion, and the spark advance. Analytical techniques for predicting the combustion efficiency and CO emissions are also presented. The predicted trends for the combustion efficiency, CO emissions, spark advance, and duration of combustion are physically reasonable and thus these modeling techniques may be of general interest. It is concluded that three characteristic delays are responsible for unwanted air/fuel ratio excursions during tip-in and tip-out. These are the time delay of the computer control system, a physical delay in air delivery due to storage of mass in the intake manifold, and a physical delay of the fuel flow which results from the finite rate of evaporation of the fuel film on the intake manifold walls (which is important during the first few cycles of tip-in and tip-out due to the large change of mass of fuel injected). The physical delay in the air flow is more significant during tip-in, the physical delay of the fuel evaporation process is more significant during tip-out, and the ECM delay is equally important during both tip-in and tip-out.