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Afm Vibration

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OK, I've been reading about AFM for about 3 weeks now since I have had the vibration problem since new.


I "think" I had a short lived "tick" sometimes when in the very cold winter months, I bought my 2008 5.3L in November 2008. I figured it was piston slap like I heard in my S10 L4 engine when very cold. Also it was new and not broken in yet.


I noticed a sound on long level roads while driving in spring that sounded like running over a rumble strip at 38-51mph. I thought it was the tire noise on some defective part of the pavement until I noticed it all over the USA when driving this summer on vacations.


Here is what it can be made to do each time I drive on a level road to 38-50 mph. For this example I picked 48mph yesterday on a 6 mile road with lights timed to stay green at 50mph.


Once up to 48 I lifted off the gas pedal to get good gas milage, I mean that is what AFM is for correct? I lift off and hold "just so tight" so that I get 24mpg and the speed is NOT dropping.


What happens is every 15-30 seconds you will get the rumble strip noise in the drive train and can feel it in the gas petal. When the noise kicks in the RPM goes from about 1400rpm to 1300rpm , losing 100rpm while the vibration happens. This will keep happening on an off as long as you are NOT losing speed.


If you let the truck lose speed, the AFM never vibrates.


Now at 60pmh I lift off and get 24mpg, and don't let it loose speed. It will "just" stay in the V4 mode and the vibration is continuous, but much higher in frequency then it was at 48mph. It can be felt like a bee buzzing on your foot in the gas petal.


I think from reading this might be from TCC engagement ?


The switching from V8 to V4 and V8 again is seamless, but it is when you are trying to get good MPG that it becomes a wreck. Coasting in V4 mode is a nice ride. Trying to get 24mpg at 50mph is a wreck.


My truck gets 14mpg driving in suburbia, 16 driving to work and back and 17mpg on long trips if you drive like a person trying to get good mpg.


Pulling a single axle trailer 6x12 fully loaded or unloaded doesn't matter; I get 8mpg to 11mpg depending on hills and or wind. This seams tearable, I friend with a jeep gets 14 pulling his much larger trailer. I have a 3.73 gear maybe this is why?


So I don't think the AFM is doing any good.


I might buy a tuner and turn it off, but I worry about factory warrentee if I do that. I understand your screwed if you tune?



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I have this between 45 and 50 in my Tahoe, its a slight growl I call it. I had the same thing in my 00 Silverado and it was the torque converter. I had the 00 reprogrammed by the dealer and it went away. Havent done anything with my Tahoe because its not that noticable. Also mine has the "growl" when it is no in AFM.

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  • 3 months later...

IMO AFM is just a placebo for those that want to buy a truck to get good mileage. I have noticed no improvement in mileage comparing my identical 2004 and 2008 GMC's. Actually is about 0.5 mil less with the 2008 in the first 20,000 miles.


A friend who is a service advisor for a big dealer told me this when I mentioned that all this hype of about mpg in this trucks was mostly bs. He said, I wish these people up front (salesman) would stop telling these customers they are going to be getting 20mpg with these trucks. We see the opposite back here in the shop when the customer comes in and says, the salesman told I would get over 20 mpg but I am only getting 16mpg.



TSB 06-06-05-001C


"Some customers may comment in a change in the exhaust note or tone when an Active Fuel Management System equipped engine is in 3 or 4 cylinder mode. Some drivers may also notice a very slight vibration in either the accelerator pedal, floor pan, and/or the steering wheel. This is a normal condition for Active Fuel Management System equipped vehicles and no repairs should be attempted."




Customer Information on Active Fuel Management


When the Active Fuel Management System is enabled, physical changes take place within the engine that allow it to run as a 3 or 4 cylinder motor with accompanying additional fuel savings. The change in exhaust tone is expected as all sound from the exhaust is the result of combustion in the engine. The ability to perceive these changes in sound are kept to very minimal levels as the Active Fuel Management System is only activated during a steady state/near steady state driving or during coast down. In both of these situations, the load on the engine is minimal and general exhaust volume (sound) is low. You may give a copy of this bulletin to your





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  • 2 months later...
Just be advised if its defeated your power train warranty will be over.


LOL. I hardly think anyone can prove changing your engine back into a full-time V8 is going to be the source of any powertrain issues under the Magnuson-Moss act.


Simple answer - advise your dealer not to plug into your ECM in the first place. They will not want to be on the hook for over-writing any gear ratio or tire size changes you might have programmed (which they are now refusing to do themselves, so your only alternative *is* aftermarket tuning).

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I'm getting about 17 now, holding my breath to see if it changes over time, but something about the 373 gear, the tires, and the ecm tune leads me to believe i won't see 20 often. i did get 19.7 on a tank on the I-90 drafting a truck doing 60 in the rain.... once with her ears back (mirrors folded in)


I don't notice the noise, but I figure it is going to make new noises running on half engine/half air pump lol.

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  • 1 month later...


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