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SUVs Chevrolet Suburban & Tahoe Chevrolet Suburban Active Fuel Management
26 messages, Last post on Jul 11, 2008 at 11:56 PM
You are in the Chevrolet Suburban & Tahoe Forum. Your Hosts are steve_ & tidester
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Replying to: ceemme (May 26, 2007 6:59 am) The dealership that I dealt with showed me the update numbers that they installed and man it was a BIG difference. GM claimed that the AFM was seamless, but, from new mine had a terrible stumble going between V8 to V4 and back. After the updates its smooth as silk! Call the GM tech line to have them open a case number for you then take it to your dealer or a dealer and have them ride with you so that they will understand whats happening. I'm also told that there is an update for the 6.0 liters that have a rough idle to them.
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Replying to: snidebj (Jun 10, 2007 4:49 pm) 
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I have a 2007 suburban with 5.6 engine. It operates fine in all situations other than it will not effectively operate in 4 cyl mode. It shifts into 4 cyl smoothly and I notice no problems nor any hint of roughness, however, it will only hold in 4 cyl mode while going down hill with your foot off the accelerator. It absolutly will not hold in 4 cly mode under any condition otherwise. On an level road at any cruise speed it will instantly switch back to 8 cyl if you ever so slightly touch the gas. I have carried the car to two different dealers and called Chevrolet customer service. The dealers said that they could find nothing wrong with the system's operation. The Chev customer service offered to located another dealer for me to take it to, but otherwise didn't seem to have any solution. Does anyone have any advice that may help?
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Replying to: mariond (Dec 24, 2007 9:04 am) I would also think any use of Ethanol blend gas, containing less energy, would force 8 cyl mode as well. You may want to try shifting to a different brand, to see if that makes any difference. |
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Replying to: ceemme (Apr 13, 2007 7:17 am) Some say its possible dealer says no. It drives me nuts. |
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When I took mine (2007 Suburban LT1, 5.3 flex-fuel) into the Chevrolet dealer with the complaint it won't stay in four cylinder mode I was told the engine and computer learn the driver's style and adjusts to the driver. It takes about 500 miles for this learning to complete. I have had my Sub since December 2007 and have been varying my driving style to figure out what gets best mileage and turns out I wasn't doing myself a favor. The vehicle couldn't figure out the nut behind the wheel.  So I took a 250 mile trip, and drove to Seattle. Normally I pride myself in maintaining speed for hills and anything else. On this trip I decided not to exceed 65 miles an hour and allow the vehicle to slow down as much as ten miles an hour for grades. This took some patience since I'm normally one of the left lane guys that passes most traffic, being passed by most traffic was different. The trip to Seattle resulted in 19 mpgs including city driving in Seattle (steep hills). Without the Seattle driving I expect I was getting 20 mpg on the freeway. A previous trip to Seattle in December I had 16 mpg but the engine had less than 1000 miles on it too. Since this gentle driving trip the 4 mpg mode now stays engaged as much as I would like it too. I've gone from an easy 13 mpg in town to 15 mpg. Bottom line keeping one's foot out of it gets the best mileage but it takes a while to notice it because of the computer's learning the driver's style. Now that I've 'programmed' mine I can have fun and get on it every once and a while and it doesn't permanently tank the mileage. |
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I have an '04 Tahoe with 5.3 L 'Z' engine. I'm getting (after all fixes I had to do) 17 - 18 MPG on interstate highway while going 77 - 78 MPH. With speed of 60 MPH I'm getting 20-22 MPG (depends who's gas I filled in). Do any of you new Tahoe, Yukon, Burb owners feel the AFM system really gives you better MPG? Does it change to 4-cyl mode when vehicle stops or does the vehicle need to be moving above certain speed before 4-cyl mode can engage? Since all 8 pistons in the engine still keep moving when engine runs in 4-cyl mode I really don't see much MPG improvement coming from it other than when going down hill or other vice coasting down. Pushing the vehicle against the wind resistance still takes the same amount of energy at a given speed no matter if the engine runs 8 or 4-cyl mode. Just curious... Arrie
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Replying to: arrie (Jul 08, 2008 5:28 am) If I cruise at reasonable speeds 60-65 on the interstate, the vehicle will run in 4 cyl mode approximately 1/2 the time. As I increase mph to over 70-75, it is less and less in 4 cyl mode as the wind resistance would be tremendous. At those speeds, you obviously would gain no mpg improvements because you'd never be out of 8 cylinder mode. In 4 cylinder mode, you are burning less gas, and therefore have less power produced by the engine. As long as the power required is less than this 4 cylinder power produced you'll stay in 4 cyl mode. I don't remember the formula for wind resistance, but it is something like increases exponentially as the speed increases...because the air doesn't have the time to move around the vehicle when you are going so fast...it just jams up in front of the vehicle. So the wind resistance increases substantially, the faster you go. I have also noticed, that if I take it off cruise control and allow the vehicle to slow down slightly on slight inclines, it will stay in 4 cylinder mode as opposed to the shifting to 8 cylinder mode to maintain any given cruise control speed. At idle, the vehicle is in 8 cyl mode, I believe for smoothness of idling....and I suspect for acceleration without delay/hesitation. I'm pleased with it. We use it only for family trips (or hauling a carload of kids around), and you can't beat the space and travel comfort. Gets much better mileage than our old 97 did, which we traded in for this. Driven prudently, the highway mileage comparatively isn't that bad. However If this was an around town beast for us (which it isn't), we'd be dieing at the pumps!
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Replying to: kiawah (Jul 08, 2008 8:26 am) What kind of MPG do you get when you run at speed that the engine stays in 4-cyl mode? As I said in my post I get 20 - 22 MPG if I drive 60 MPH and I have 92 000 miles on the engine. Personally I think this AFM is mostly a hoax to get people buy them trucks as it really can not save gas other than when engine is IDLING or you are coasting down the speed or if the engine is so very poorly designed that it wastes a lot of fuel that is sprayed in the cylinders (which it could very well be too). As I said, for moving the truck at any speed takes the same amount of energy to do, no matter how many cylinders are in use. AFM would make best sense if it would switch to 4-cyl mode when you stop at traffic lights etc. These big engines do not need all 8 cylinders to turn themselves around but perhaps it is about the smoothness of idle and they keep it on 8-cyl mode. So, what is your MPG when you go constant speed of 60 MPH on highway? Arrie
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Replying to: arrie (Jul 08, 2008 1:42 pm) I understand your thought process, but it doesn't necessarily work that way. You are correct for any given speed of the vehicle, it takes a specific amount of energy or HP to make that happen. The problem though, is it would have to be connected to an infinitely variable transmission, so the engine could reduce it's RPM's down to produce just that HP....while maintaining the speed that you wanted. Because of the transmission limitations of the number of gears, when you pick and drive a specific mph...your engine will be turning at a specific resulting RPM as dictated by the gearing, and you would be producing the exact HP or MORE than is necessary. I'll attach an engine HP and torque curve chart, happens to be for a Toyota we have but you'll get the idea. Any given engine's HP produced, is dependant upon the RPM's that it is turning. Internal engine design (like bore, stroke, fuel, etc) dictate the power and torque curves it produces. So when you pick your speed that you are running, and the transmission is in a specific gear, your engine will be turning at a specific RPM. The HP produced will be a given for that engine at that RPM, and most likely will be more HP than is needed. If the HP is more than is needed, you are wasting fuel. The AFM, will effectively give you two power curves for the same engine. A power curve when delivering fuel to all cylinders in 8 cylinder mode, and a power curve when delivering fuel to only 4 cylinders. This gives the engine the ability to drop down to 4 cylinder mode, if the HP of the 4 cylinder curve at that specific RPM is enough to move the car at the speed you have chosen (fighting against the road grade and wind resistance). Engine HP and Torque curves
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