We have developed a new wall wetting model to predict the transient Air/Fuel ratio excursion in a port fuel injected (PFI) engine due to changes in air or fuel flow. The quasi-dimensional model accounts for fuel films both in the port as well as in the cylinder of a PFI engine and includes the effects of back-flow on the port fuel films to redistribute and vaporize the fuel. A multi-component fuel model is included in the simulation; it gives realistic fuel behavior and allows the effects of different fuel distillation curves to be studied. The multi-component fuel model calculates the changing composition of the fuel puddles in the port and cylinder during the cycle. The inclusion of an in-cylinder fuel film allows the model to be used for cold start conditions down to 290 K. The model uses the Reynold's analogy to calculate the fuel vaporization process and uses a boundary layer calculation to solve for the liquid film flow. The fuel vapor and liquid dynamics are calculated every crank-angle degree using the instantaneous air flow conditions in the port and cylinder. We compared simulation predictions with the experimental transient Air/Fuel trace from a 1.6 liter, in-line-4-cylinder (14), single-over-head-cam (SOHC) engine and a 5.4 liter SOHC V8 engine over a range of speeds and temperatures. The predicted results agree well with the experimental results. We also compared simulation predictions with the 1.6 liter engine for the case of heavy carbon deposits on the inlet valve.