Hi Scotty: I have a 2007 Chevy Impala LT with the 3.9L MFI V-6 SOHC Automatic and what they call Active Fuel Management system. (It switches between 6 cyl and 3 cyl. Just a little over 98,000 miles on it. Oh, and 17" aluminum machined rims. We do a lot of round trip driving of about 15 miles and then an occasional trip of about 60 miles. Chevy claims gas milage of 20 MPG city and 29 MPG hwy. I haven't tried a long trip yet, but get only 14-16 MPG city. On the dash there is an instant MPG shown. As I start from a stop it shows 7 MPG and 6 cyl indicator. As I continue to drive at a constant 35- 40 mph it changes to 40-60 mpg and a 3 cyl indicator. There are two pretty steep inclines on one of our shopping trips and when going up the hill 10-15 mpg/6 cyl shows. On the way back the down hill slope is a much longer distance and the car coasts, at which time the indicator displays 99 mpg/ 3 cyl. In my estimation more 60 mpg/ 3 cyl is indicated the the lesser values. However, at the pump and after putting about 100 miles of driving before filling up the real numbers come out to 14-15 mpg. How can it be so low? Thanks. Rick.
That's hilly terrain for ya. You would probably see closer to 20 or more if you just drove flat, but AFM is useless going up hills, and a 3.9L engine is no fuel sipper. Coasting down will never make up for the fuel you burn going uphill. That's just the law of the universe.
Thanks for your comment.
Welcome to the forum. This might be a good question for a pro with one of Scotty’s “fancy scan tools” (a regular OBD reader like you’d get off Amazon is not likely to tell you anything).
I think you’ll get a consensus here that the advantages of cylinder deactivation are mostly vaporware. Looks good for EPA testing but underperforms in real life. And usually just shortens the life of the engine. Whether reflashing it to turn it off will make a difference with a car that old, I don’t know.
Thanks for your comment
Because car manufacturers lie to your face. And the AFM feature destroys engines.
Also realize that the claimed mileage ratings are what was observed under optimum test conditions, and now your Impala is 15 yrs. old and nearly 100k miles on it. But that is the case with the epa mileage shown on the window stickers for all car manufacturers.
