The other day, while browsing discussions, I came across a comment somebody posted which I have been pondering all day and have not been able to figure out, mechanically. It’s nothing new, as I have seen the same statement made by a few other posters, including regulars, since I started posting in 2007, but never gave a tremendous amount of thought to it, but for some reason, it’s bugging me now.
The jist of the statement is this: on any fuel injected car made in the last 15-20 years, during deceleration, the ECM completely shuts down the fuel system to save fuel because the engine does not need fuel to run unless under load. I cannot understand how this can be so. It seems to me that the engine would promptly stall if the fuel were taken away due to the need for combustion to overcome resistance from friction, compression, and the valvetrain. I understand that the momentum of the vehicle could keep the engine turning as long as the clutch is engaged or the torque converter locked, but it seems like this would be difficult for the electronics to manage given such variables as the possibility that the driver may shift to neutral or depress the clutch, which would cause the engine to stall if fuel were not demanded immediately as this happened. It also seems like this would make for a pretty rough ride under deceleration. I considered turning off the ignition in my truck while in gear under deceleration (M/T) to see if I noticed a difference, but chose not to.
So, the question is: is this fact or fiction, and if it’s fact, how does it work while maintaining the driveability of the vehicle and preventing constant stalling?