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2007 GMC Sierra 5.3 SLT NNBS


I'm pretty focused on keeping this truck is the best possible running shape, as I can. That being said, when having my vent valve replaced, I had the pcm updated. It seems to run slightly better, less idle shake, but I could be fooling myself. However, my question is with regards to mileage. Prior to the update, I was getting about 18.3L/100km (yes, Canadian) now I'm at about 20.6L/100km. Does this make sense to anyone as to why this would be negatively affected? I haven't changed my driving habits or style and the physical being of the truck has been the same throughout. Same tires, same wheels, etc.


Does a pcm update take some time to re-learn operations (it's only been about a week and half since I had this done)? Will it go back to the same efficiency as before?


Can the original factory pcm configuration be restored?


Any input to this would be very much appreciated.


Thanks kindly, eh.

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is it possible that your area is now switching back to "summer blend" gas? I'm from the deep south and we don't have variations like up north, but I've heard of people commenting on achieving different fuel economy numbers.


the computer update may have tightened up some details in the idle, that is possible. also, yes, the computer does learn - unlike some people try to insinuate the computer is constantly learning. You driving style makes a big difference in economy and one tank of fuel is not enough to judge a "change".

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The change to summer blends, if canada does that sort of thing, can change fuel consumption. My experience is that on summer blends, i average 14.5mpg with the type of driving I do. Summer blends in the US tend to increase economy, all things being equal.


On winter blends, mine can be low 13s, again, all other things being equal.


Yes, PCM updates could change consumption as well. All depends on what the update is/was. IF timing is impacted via the update, yes, it will impact economy. Idle quality is a function of many things, including timing, rpm at idle, and all kinds of other stuff. Quite possible that an idle quality improvement via the PCM could, indeed, impact economy elsewhere.


Engine operability is a plethura of compromises throughout the operating map. All depends on what their focus was in determining the engine control characteristics.


Back in the 91 time frame, Ford's number on customer criterion (wrt the power train) was IDLE QUALITY of the old 302. Yes, I saw the data, and it was true. Ford went so far as to sacrifice ALL KINDS of operability elsewhere via the cam to get a smooth, non-shakey, idle. Killed some mid-range performance, but it idled smooth. Good? Bad? I'll let you make the call. But, often, fuel economy is trumped by other customer wants --- ESPECIALLY for the truck market.

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