Question: My truck is a 2011 Chevrolet Silverado Crew Cab with the Vortec 5.3-liter V-8 with Active Fuel Management and a 3.42:1 rear-axle ratio. The EPA's average estimated fuel economy is supposed to be 15/21 mpg city/highway. It now has about 3500 miles on it. The tires have plenty of air; it's not loaded or going against the wind; and I'm using regular gas. Changing to midgrade gas didn't help. I'm getting 15 mpg city and 16 mpg highway. The owner's manual indicates that, if you're in cruise control at steady speed on level road (not under any extra load), Active Fuel Management should kick in and put the engine into V-4 mode. One would think that would include normal highway speed limit at about 70 mph. I don't understand it, but all the mechanics say AFM won't do that. I've taken the truck to the dealers, but they can't find any trouble codes to tell them what to do. The dealers I have spoken with don't seem to know much about the AFM; if it doesn't show a trouble code, they don't know how (or have the time) to trace the problem. They say this is normal. Do you think it may be possible (with the manufacturer computer settings) to get the AFM to stay in V-4 mode up to 73-75 mph? I just can't drive on the open road at 55-60.
Answer: If you want the highest fuel economy out of your truck, that's what you're going to have to do. It drops significantly above 60 mph. As far as Active Fuel Management is concerned, the dealership technicians are most likely correct, and the system is performing as designed. I've heard similar complaints. I have personally experienced cylinder deactivation not engaging at high speeds (even under light load). And, no, technicians cannot adjust the timing of cylinder deactivation. The parameters are controlled by powertrain control module software programmed by the engineers. I also know of other brands of vehicles experiencing engine damage (cylinder wall scoring) due to cylinder deactivation being engaged for long periods of time (perhaps at higher engine speeds). The engineers themselves aren't quite sure why, but are continuing to perform software reprogramming in an attempt to alleviate the problem.
At the dealership, this is as far as a technician could go in an attempt to fix the problem, or prove there is none: After checking for trouble codes and normal engine data with a scan tool, the tech should look for any relevant technical service bulletins or service news articles, and after that be sure the powertrain control module has the most recent software updates. The next step would be contacting GM technical support. Tech support would search its history database for similar issues, and possibly request a "snapshot" of data while Active Fuel Management is being engaged. Or, good chance, they may also say it's operation is normal. I wouldn't invest a lot of time in it.
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