Question: My home is in northwestern Pennsylvania and just about in any direction traveled, hills have my 2011 1/2-ton Silverado 5.3-liter V-8 constantly shifting from eight to four cylinders. Generally, my truck's mpg rarely exceeds 18. I always drive within legal speeds, and power stops and jackrabbit starts just aren't in my driving repertoire. My truck is a four-wheel drive with a limited-slip differential and 3.08:1 axle ratio. Also, whenever it drops into four-cylinder mode, the truck almost seems to lug -- perhaps the ratio is too high for the truck's weight, which is GM's fault for offering the choice. Yet I rarely haul anything other than hunting or fishing gear and the truck also carries a full-time, cab-high fiberglass cap. Would the truck do better mileage-wise if the engine ran continuously on eight cylinders? And is it possible to change a chip or something else to make continuous eight-cylinder operation happen?
Answer: No, and yes. GM's Active Fuel Management, as with all cylinder deactivation systems, does improve fuel economy. But the amount of fuel savings between V-8 and V-4 mode is somewhat of a gray area because of all the variables. AFM is intended to apply under higher speed, light-load conditions. Going uphill (or pulling a boat) increases load and deactivates the system. Downhill will put the engine back in V-4 mode as designed. A hilly trek will produce a lot of in and out. The lugging you've described is most likely a normal characteristic of operation. GM has published a technical service bulletin describing "change in the exhaust note or tone" and "very slight vibration in either the accelerator pedal, floorpan, and/or the steering wheel" as normal conditions during cylinder shutdown. Another note on cylinder deactivation: It is strongly suggested that these systems can produce cylinder wall/piston ring damage because of its effect on cylinder wall lubrication at the disabled cylinders. It seems most manufacturers have discovered this problem, and are doing their best to tweak control module programming as a fix. An example: GM's Active Fuel Management operates in V-4 mode for 10 minutes, then back to V-8 for one minute (giving the cylinder walls a break), then back to V-4 for 10, and so on. There are aftermarket reflash units that allow you to disable cylinder deactivation. Hypertech makes the Max Energy Power Programmer Part #32501 to fit your Chevy truck. But keep in mind, GM's TSB 09-06-04-026G stipulates that aftermarket powertrain control module calibrations have produced engine, transmission, and catalytic converter damage, and therefore GM reserves the right to end powertrain warranty with confirmation of a non-GM calibration (software) ID.
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