Can anyone explain the principle behind defueling, is this to prevent damage to the drive train, is it normal in the 6.4, what does it feel like and how long will it typically go on until the system returns to normal

 

I don't know all the parameters on the 6.4 w/ de-fueling, but mostly to protect the engine. I know when my truck is cold (when first driving in cold start ups). My power is some what reduced until the temp guage goes up. Then it is a different truck on throttle control and WOT getting on a freeway or merging.

I thought I read somewhere that Ford put this programming in to protect a cold engine until temp reach around *140.... Someone can correct me if I'm wrong.

 

it is to protect the drive train when shifting gears. To me it feels like if you had a manual and you were to let off the gas while shifting.

 

Mark Kovalsky would be a great one to answer this one if he's on here. My understanding is like what senix said. It protects the transmission during shifts so that the peak torque isn't transferred to the transmission internals.

 

In the bigger trucks they refer to this as "SEM" or shift energy management. It allows them to use a higher power rating engine than they could have otherwise used with the transmission/drivetrain.

 

I think both answers are correct. It definitely does it to protect the transmission as Senix said. When the engine is cold it will "limit" some fuel so that isnt exactly the definition of defueling (or at least its different than the defueling for the shift strategy). These trucks also defuel on initial acceleration to limit the amount of emissions or soot.