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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
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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Replying to: ceemme (Apr 13, 2007 7:17 am) 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). |
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Replying to: ceemme (Apr 13, 2007 7:17 am) 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!
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Replying to: snidebj (Apr 25, 2007 2:01 am) 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).  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!
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Replying to: jerrywimer (Apr 25, 2007 3:26 am) 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".
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Replying to: snidebj (Apr 26, 2007 4:31 pm) 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!
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Replying to: jerrywimer (Apr 27, 2007 4:29 am) 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.
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Replying to: snidebj (May 10, 2007 12:20 pm) 
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Hey guys, 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 !!!
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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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