The “Sac” is a small volume within the fuel flow path of an electronic fuel injector. In this study, it is defined as the volume between the valve seat (fuel shut off point) and the entrance to the final metering orifice of the injector. This sac causes fuel injectors to deliver uncalibrated excess fuel when the engine is operated under closed throttle, high manifold vacuum conditions such as vehicle decelerations or idle. This paper describes a simple mass balance model used to predict the effect of the sac volume on injector fuel delivery under extreme operating conditions. The model prediction compares directly with experimental results for injectors with different sac volumes.