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Chevrolet Suburban Active Fuel Management
While the switchover between 4 and 8 cylinders is seamless, i.e., no bumps, etc., I do notice a slight change in how the engine sounds and "feels". For example, while climbing a long hill that bearly requires the V8 mode, the engine is absolutely quiet. However, after topping the hill and the V4 mode kicks in, I hear a definite change from quiet to a somewhat labored sound accompanied with a slight vibration in the gas pedal. Then, as the engine switches to the V8 mode, it's quiet again until the call for V4 begains the cycle over again. This is quite annoying and the dealer diagnostics has revealed nothing. Vehicle is 2007 LT2 with 5.3L V-8. Also, I've discovered no change in gas mileage. Sorry, I forgot to actually ask a question. Does anyone have a similar experience? If so, what is the solution. ( dealer tells me there are no official GM bulletins on the problem)
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As far as a change in gas mileage goes- 4 cylinder mode DOES increase mileage over 8 cylinder mode during level or slightly uphill grades (assuming you can get yours to remain in 4 cylinder mode on the grades- my 4.10 rear-end Av does, though lots of other owners report their vehicles won't). I HAVE noticed that downhill grades seem to be unaffected regardless (drifting). I'm not 100% certain of this, but I believe there's some sort of fuel cutoff in effect even in V8 mode when the vehicle is drifting (meaning it actually uses less fuel than at idle, NOT that it shuts the engine off completely). This nearly or completely negates the difference between 4 vs. 8 cylinder operation on downhill grades.
(And yes, I've spent enough time observing the 'instant' readout on differing grades- my daily drive includes lots of variation, thanks to currently living over 30 miles from work in the western NC area, and has a 6+ mile stretch of STEEP grade as part of it)
The only real complaints I've seen concerning the 4 vs. 8 modes with these vehicles so far didn't have anything to do with the sound so much as the frequency 4 cyl mode gets engaged (more specifically, lack of it, inability to get the vehicles to stay in it, even on level roads for some, etc.), and also to an extent, whether it really has much effect. I tend to think the (lack of) fuel economy impact is related to the first issue (inability to keep it engaged for any reasonable amount of time).
When in V8 mode its smooth as silk and powerful but when in V4 mode its rough and there seems to be a "miss" almost like the timing is off.
Annoying at best is what I give it!
When idling in the driveway, the idle is rough.
It doesn't get any better gas mileage than my 05 Cadillac Escalade or the other 2 Avalanches that I have owned in the past.
There has got to be some way to disable this function that they tried in the early 80's called the V8-6-4. Remember that one abortion?
Please if anyone has any info on how to deactivate this "Active Fuel Management" let us know!
Thankyou!
1) The "constant" switching back and forth is fairly normal, in my experience.
2) I never notice the miss in V4 mode (then again, mine's a 5.3).
3) The rough idle isn't related the the V4 / V8 thing- one of the problems many folks have with the way GM implemented this feature is that they decided NOT to enable V4 mode below a certain speed threshold (mayb 20mph?). That means that your rough idle is entirely in V8 mode. So disabling the feature won't fix this problem.
4) I don't know the weight of your previous 3 vehicles, but the 07 Avalanche weighs on order of 5700 pounds empty. That, and there was an increase in power over the previous engines. Keep in mind that this is comparing the 5.3l engines, and you have the 6.0l. If you're getting NO WORSE than you got with your previous 2 Avalanches you're doing pretty good, since the 6.0l wasn't an option in them.
5) AFM isn't the same thing as V8-6-4, even if the end result is similar. Other than the fact that both deactivated some cylinders to save fuel, the way they do so, and even the available modes, are different.
I wonder if DCX / Honda owners have as many complaints with their cylinder deactivation, whether concerning how it operates (feel) or how much it really benefits their fuel economy (if at all). :confuse:
For the record, I'm breaking 20mpg regularly now (mostly highway) with my 07 5.3l 4x4 LTZ. I attribute quite a bit of that the the AFM actually working. My previous vehicle was a 2004 Silverado ECSB 4x4 Z71 (~5200 pounds) with 3.73 rear end and 5.3l rated at just under 300hp. The taller gearing allowed that lighter vehicle to achieve close to 19 most of the time and just over 20 once in a blue moon. Given that my Av weighs a good 500 pounds more, and that its engine received an additional 20hp or so (and roughly the same amount of torque), the fact that I'm doing the same or a bit better tells me that the AFM is doing fine by me.
You might look into contacting one of the companies that provides custom tunes for our vehicles. I understand that many have now started doing this for the 07's with AFM. From some of the reading on the Avalanche forums I visit elsewhere, at least one may result in the AFM ceasing to function, as the company in question stated that they haven't had time to research the tune parameters associated with it enough yet.
Good luck!
Apparently this AFM system is getting alot of attention with the "reportedly seamless" transition between V8 and V4 mode. Mine is not seamless with the 6.0 liter. Far from it! Plus there are others out there like wjith the same symtoms. Most would like or are requesting to disable the AFM.
I had my 6.0 liter Avalanche at the dealer and they report that its in normal operating conditions! Hard to believe but..........if GM has designed this "AFM" to have operating conditions such as what I have, then they have some serious engineering problems. Besides, my neighbor has the 5.3 liter and we took them down together. His engine operates as bad or worse than mine.
The dealer took 2 Avalanches off the lot and they operated smooth as silk! Big margin for "normal operating conditions".
Sorry you're having problems though. I still suggest looking into getting a PCM tune. Besides the potential power and fuel economy boost (the reasons most everyone else gets the tune), there's the chance that the new tune will reduce or eliminate the switchover to V4, as I said earlier. Since you've got an Avalanche, I highly recommend looking up the Chevy Avalanche Fan Club of North America and its forum (won't link it here, to keep the mods from smacking me around). The one or two experiences with tuning on an 07 that mentioned the AFM being affected were on that site.
Good luck!
The dealership tech and the GM tech loaded in 10 updates to address the situation.
Yahooooooooooooo! I would say that after the updates and driving another 500 miles or so, that the AFM switches almost as GM claims as "seamless". Man, am I glad that they got it fixed because it was annoying as hell.
My 5.3L LT2 acts just the same as Jerry's 6.0L prior to the software updates. However when I took my 2007 Suburban back to the dealership, they tell me that there are no reported problems with AFM and consequently no software updates available. I wear two hearing aids and not only hear the change in the exhaust noise going from 8 (smooth) to 4 (rough), I can feel the vibration in the footfeed.
What to do !!!
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.
Does anyone have any advice that may help?
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.
Some say its possible dealer says no. It drives me nuts.
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.
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
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!
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
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
You get the same MPG as me on highway so that AFM does not really save fuel on highway. It could save if it was made to take 4-cyl mode every time power demand from engine allows and perhaps this happens during highway driving but as you say it is kept in 8-cyl mode when the engine is on idle you don't get the savings while driving in city traffic. Idling in 4-cyl mode would probably be the biggest benefit from that mode. Remember a few years ago when police departments in the country changed from using Crown Victoria to Impala just to have a smaller 6-cylinder engine for their idling as that is what police card do a lot.
About the engine curves. Yes, have seen a couple. Power is simply torque multiplied by engine revolution speed. As the engine speed is a linear straight line as the horizontal axis of the graph the torque curve is not. Torque curve is the one which tells how good the engine is and I strongly disagree with your statement that 4-cyl mode provides the engine with another torque curve.
Torque curve is controlled by engine design as how gases flow in and out the engine and how ignition spark and intake / exhaust valve timing is set up. A big thing with this is with valve timing and new high end engines now use variable valve timing for both intake and exhaust valves. This technology certainly has not reached Tahoe engines yet. Also intake and exhaust manifold design has a lot to do with gas flow handling.
Valve timing is done to improve that torque curve to maximize the area for best torque. The problem with torque curve always is that it drops off at certain point and variable valve timing has been found to be an excellent toll to improve this.
It still is the fact that no matter what the power, it is the FORCE, i.e. ENGINE TORQUE that makes the car move. Power just tells how much of that torque can be taken out from the engine in a time unit.
Spraying fuel in 4 cylinders instead of 8 does not change the engine properties for gas flows or valve timing etc. It just basically means that you will be spraying twice as much fuel/cylinder in those 4 cylinders to keep the vehicle going at set speed compared to using all 8 cylinders.
Yes, and the two mode system is one more thing that needs service and repair...
Arrie
I never said the engine would have two torque curves....I said it would have two different power (as in horsepower) curves. When running in 4 cylinder mode, you'll have 4 power strokes per 2 engine revolutions. When running in 8 cylinder mode, you'll have 8 power strokes per 2 engine revolutions. You're burning twice as much fuel in 8 cylinder mode.
It just basically means that you will be spraying twice as much fuel/cylinder in those 4 cylinders to keep the vehicle going at set speed compared to using all 8 cylinders.
The engine does not spray twice as much fuel/cylinder. The piston moving up and down is a given volume displacement. You do not vary the mixture in the cylinder to double the hp. If you doubled the amount of gas in the same volume of air, you would run extremely rich, the mixture would not burn completely, and it would clog up the cat converter. If you leaned out the mixture by not providing enough gasoline in that volume of air, you'd burn out the valves.
You seem to have so good handle of this on your own level I'll let it be.
but as an absolute last comment from me on this...
It just is that the car takes certain amount of force (energy per mile) to push against wind resistance (and rolling resistance) at given speed. As long as that force (and energy) comes from the gasoline in the car's tank the same amount of that gasoline is needed to push that car with that given speed REGARDLESS OF HOW MANY CYLINDERS OF THE ENGINE IS DOING THE WORK!
Arrie
It will take the some force and energy to move that vehicle forward cruising at 60mph (let's call it X, or better yet just assume /define it to be 65HP). I think we both can agree to that.
Now lets talk about what energy the engine 'actually' produces. At 60mph, in high gear, let's for the sake of discussion say the gearing puts the engine RPM at 3000rpm. Using the prior 2GR Toyota chart, at 3000 rpm the engine is producing 140 HP. (Yes I know the Tahoe doesn't turn at 3K at 60mph, but we're using the Toyota charts since I already posted them, and the gearing for both are higher). Even though the vehicle only needs 65HP to move thru the air and down the highway, the engine is producing MORE horsepower than is needed. That extra energy (heat) is wasted and expelled (through the radiator and exhaust).
Now what APM can do, is shut down 4 of the cylinders, and not fill them with fuel mixture. The engine's power curve now, since it's burning half as much fuel for any given RPM, would be approximately half. So now the engine at 3000 RPM is producing 70HP. 70HP is still more than the 65HP needed to move the vehicle thru the air at 60mph, so the speed is maintained. It doesn't slow down, or downshift. There is now a closer balance between the HP needed to move the vehicle, and the HP actually produced, so there is less wasted energy.
Why then build engines that produce 140HP? Because when you are starting a vehicle from a stop sign, or carrying a heavy load, or driving up an incline......you need much more than the 65HP that the vehicle might need cruising on the flat interstate.
First I need to correct my statement that the 4-cyl mode does not provide a new torque curve. Of course it does as the torque roughly is half of the 8-cyl mode at given rpm but what I mean is it does not provide a useful additional torque curve and I stay behind that. The only way 4-cyl mode could be helpful is that if it would take the 4-cyl mode every time the power requirement from the engine allows to do so. From what I have learned about it this is not how it works.
Now about the torque curve. The engine does not produce torque (and power) by the curve when you normally drive the car. It only produces it when you pull the maximum torque from the engine, i.e. THE TORQUE CURVE ILLUSTRATES MAXIMUM TORQUE OUTPUT THAT THE ENGINE CAN PRODUCE WITH CORRESPONDING ENGINE REVOLUTION IN THE CHART!
When you normally drive say that 60 mph speed the engine provides exactly that amount of torque (and power) to keep the car going that speed. If it would provide more than is required the car's speed would increase unless you step on brake pedal or other vice cause more "friction" for the engine to work against. The engine does not provide the torque (power) what the torque chart shows for that engine rpm unless the engine's performance is on the max limit for that engine rpm.
Seems like you were a perfect customer to buy one of these AFM hoax vehicles.
...lol...
This was my last post on this topic with you.
Arrie
I found this reference for you if you'd like to read up on the theory of how Automobile Engines work. It's a little long and detailed for easy casual reading, but it covers the physics at work in an automobile internal combustion engine.
Physics in an Automobile Engine
This one's an easy read. It's written by Barry Parker, a physics teacher at one of our local colleges.
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