New to Acitve Fuel Managment, leave it off or turn it on

Big5 · July 31, 2016

I bought a used 2013 GMC Sierra 5.3 ext cab last year. Bought it from an older gentlemen that only put 6K on it. I've put 10K on it during the past year. I get 13.5 mpg on average, which is not a big deal for me as the 2000 GMC sierra 5.3 I had for 15 years got 14.5 mpg on average. The option list for my truck includes AFM. Since the AFM has never been active since I've owned the truck I'm guessing the previous owner turned the AFM off. Is it worth the trouble of turning the AFM on? I have friends that have the newer GMC/Chevy 1500 series, 5.3 and they are telling me they are getting 20+ mpg with AFM on the highway. Is this experience consistent.

BlackZ71Silverado · July 31, 2016

How did the owner disable the AFM? It's not something you can just turn on and off, that's not an option. You can either do it with a tune, or that Range trickery gadget you have to keep plugged into the OBD2 port. If you decide to have it turned off, do it through a good custom tune.

The 2013 model year is updated over the earlier models though, haven't really heard of the 13's having oil consumption issues.

Big5 · July 31, 2016

I called my local GMC dealership and spoke with their service department about whether AFM could be turned off. The service manager I spoke to said he didn't know and he suggested that I contact GM. The dealer has my trucks info in their system, so I figured it wouldn't be difficult for them to help me figure this out. Looks like I was wrong. I haven't called GM yet. Based on what I've read on AFM experiences from other members on this forum, I'm certain that AFM is not currently active on my truck. But based on the Vehicle build Information sheet it specifically calls out PDU - Power tech package, vortec 5.3L V8 engine w/Active fuel management.

SnakeEyeSS · July 31, 2016

Might have a tune on it.

BlackZ71Silverado · July 31, 2016

Unless the truck you bought is tuned, AFM is active. Again, It can not just be turned off.

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shane06fl · July 31, 2016

The AFM can only be turned off with tune or the Range gadget. I think some OBD turners can also turn it off.

I have a 2013 5.3 and it used about 1qt for every 4000 miles. I buddy gave me his Range and the oil use stopped. No real change in Fuel economy either. What I did notice is the weird down shift coming to a stop at about 5mph disappeared.

The AFM really isn't hurting much in 2013 and newer engines but I don't like getting low on oil. Monitor how much oil it uses and go from there

Big5 · July 31, 2016

i don't burn any oil at all. I use mobile one full synthetic and I change it every 5K. I guess I can live with getting 13.5 mpg without the AFM. Sounds like the side affects of turning it on may not be worth it.

MaverickZ71 · August 1, 2016

You can tell if your AFM is really turned off by watching the instant mpg screen on your DIC--if you are in 'D' for drive gear and coasting at highway speeds, you can see it switch into V4 mode. If it never shows V4, you AFM has been turned off via an altered PCM tune or Range V8 module plugged into your OBDII port.

If your AFM is off, leave it off, or you'll probably have oil usage problems later. AFM might gain you one MPG, if that, but it will cost a lot to have your engine rebuilt with new lifters and piston rings, and it's not worth it. You are not going to see 20mpg+ overall on a tank of fuel driving a GMT900 truck, even if it is 2wd with 3.08 gears, unless you are coasting downhill with a heckuva tailwind.

And don't bug GM about whether your AFM is off or on. If they find out your AFM is turned off via an altered PCM tune, they'll void your warranty.

Kurt1234 · August 2, 2016

Local dealer said they won't turn off AFM due to emission/federal law.

arcasey · August 2, 2016

If your AFM is off, leave it off, or you'll probably have oil usage problems later. AFM might gain you one MPG, if that, but it will cost a lot to have your engine rebuilt with new lifters and piston rings, and it's not worth it. You are not going to see 20mpg+ overall on a tank of fuel driving a GMT900 truck, even if it is 2wd with 3.08 gears, unless you are coasting downhill with a heckuva tailwind.

And don't bug GM about whether your AFM is off or on. If they find out your AFM is turned off via an altered PCM tune, they'll void your warranty.

I am not 100% in agreement with the statement of you can't get 20 MPG on a GMT900 vehicle. i previously owned a 2011 Sierra crew cab Z71 4x4 with the 5.3L and 3.42 rear end and I got 20.5-21.5 MPG on the highway consistently on the long highway trips I took.

However, I will agree that you won't get much better than 16 in the city on those trucks. As far as the AFM, I had a tuner on that truck and I drove it with and without AFM and never really could see more than 0.5 MPG gain on my truck. Also, unless you are just completely driving in the city and stomping on the gas every time you take off, I would venture to say that there is something wrong, I don't know what, but that truck should easily get at the very least 14.5-15 MPG just in the city and should get around 16 combined MPG. Is that 13.5 MPG calculated by hand or is that the value you are reading from the dash display?

I would look at it like this: That truck should still have a warranty. Any authorized dealer should be able to hook it to a computer and tell you if the AFM is cutoff. If it is, the truck either wasn't right when the previous owner purchased it, or it has been tuned. Turning AFM off should not in any way harm or hurt any component or aspect of the vehicle. I won't say with 100% certainty because I don't know all the facts but I am pretty sure that it is actually better on the engine and its components for AFM to be turned off. So, if it has been tuned, your warranty cannot be voided unless the tune is responsible for causing whatever part is being covered under warranty to have failed (I reference the Magnuson-Moss Warranty Act). The dealership should be able to fix whatever is wrong with the truck (assuming something is wrong and causing your poor fuel mileage issue) under your warranty and tell you if AFM is turned off and get it corrected if it is.

MaverickZ71 · August 2, 2016

I am not 100% in agreement with the statement of you can't get 20 MPG on a GMT900 vehicle. i previously owned a 2011 Sierra crew cab Z71 4x4 with the 5.3L and 3.42 rear end and I got 20.5-21.5 MPG on the highway consistently on the long highway trips I took.

However, I will agree that you won't get much better than 16 in the city on those trucks. As far as the AFM, I had a tuner on that truck and I drove it with and without AFM and never really could see more than 0.5 MPG gain on my truck. Also, unless you are just completely driving in the city and stomping on the gas every time you take off, I would venture to say that there is something wrong, I don't know what, but that truck should easily get at the very least 14.5-15 MPG just in the city and should get around 16 combined MPG. Is that 13.5 MPG calculated by hand or is that the value you are reading from the dash display?

I would look at it like this: That truck should still have a warranty. Any authorized dealer should be able to hook it to a computer and tell you if the AFM is cutoff. If it is, the truck either wasn't right when the previous owner purchased it, or it has been tuned. Turning AFM off should not in any way harm or hurt any component or aspect of the vehicle. I won't say with 100% certainty because I don't know all the facts but I am pretty sure that it is actually better on the engine and its components for AFM to be turned off. So, if it has been tuned, your warranty cannot be voided unless the tune is responsible for causing whatever part is being covered under warranty to have failed (I reference the Magnuson-Moss Warranty Act). The dealership should be able to fix whatever is wrong with the truck (assuming something is wrong and causing your poor fuel mileage issue) under your warranty and tell you if AFM is turned off and get it corrected if it is.

Mileage always calculated by hand. Can't trust the DIC, as it has always been about 20% overoptimistic. Fuel at the same station/same pump most of the time. Bone stock 5.3L LC9 with MYC 6L80E 6-speed automatic, auto 4WD, and 3.42 gears. Since new, average 12 mpg in town and 17 on the highway, if lucky. And I live in a flat state and drive easy. Window sticker said 14 city/20 highway. Like they say, your mileage may vary.

What I said was 20mpg+ OVERALL (full tank, both city and highway driving) was not likely in a GMT900. I fully realize somebody might even see 30+mpg indicated on the DIC during highway driving (downhill or coasting), but that won't last very long.

Good luck arguing that Moss-Magnuson Act with your local dealers. You can try to sue GM corporate (good luck with that!), but local dealers have received orders from GM corporate in recent years to not complete any powertrain warranty work on vehicles with altered PCM tunes. I've personally seen my dealer hook their computer up to a vehicle brought in for warranty work, identify that the vehicle had a performance tune, unhook it, back it out, and tell the customer there was nothing they could do. When I inquired about that, I was informed that if an owner really insists, then they'll perform the repairs, and then hand you a fat bill for the work before you get your keys back.

2017SierraSLT · August 2, 2016

I have been satisfied with the mileage from my '13. Trips usually net 20 plus. Towing a 4200 lb car and 1800 lb trailer netted 11.5 MPG over 1900 highway miles. Lifetime per Fuelly is 14.7 mpg. My daily commute is filled with urban driving with several traffic lights.

Big5 · August 3, 2016

I appreciate everyone's experience. I've never seen anything come on on the dash for AFM. All of my MPG are done manually. 13.5 is all I get and I check it almost every time I fill up. When I tow my 20' Skeeter I get about 11 mpg. I've read enough horror stories about AFM to make me think twice about getting it turned back on. Of course I'm assuming it was at some point turned off prior to my purchasing the truck. But there is no doubt that the AFM is currently turned off. There is nothing on my display that shows mpg while I'm driving.

dirtfan21 · August 3, 2016

I'm curious, is this truck a WT, SLE, or SLT? Only reason I ask is if you have the WT option package you may have a 4.8L which does not have AFM all together. If it is a 5.3L and a SLE or SLT then you have to select through the DIC buttons to see the v8 or v4 mode.

arcasey · August 3, 2016

Mileage always calculated by hand. Can't trust the DIC, as it has always been about 20% overoptimistic. Fuel at the same station/same pump most of the time. Bone stock 5.3L LC9 with MYC 6L80E 6-speed automatic, auto 4WD, and 3.42 gears. Since new, average 12 mpg in town and 17 on the highway, if lucky. And I live in a flat state and drive easy. Window sticker said 14 city/20 highway. Like they say, your mileage may vary.

What I said was 20mpg+ OVERALL (full tank, both city and highway driving) was not likely in a GMT900. I fully realize somebody might even see 30+mpg indicated on the DIC during highway driving (downhill or coasting), but that won't last very long.

Good luck arguing that Moss-Magnuson Act with your local dealers. You can try to sue GM corporate (good luck with that!), but local dealers have received orders from GM corporate in recent years to not complete any powertrain warranty work on vehicles with altered PCM tunes. I've personally seen my dealer hook their computer up to a vehicle brought in for warranty work, identify that the vehicle had a performance tune, unhook it, back it out, and tell the customer there was nothing they could do. When I inquired about that, I was informed that if an owner really insists, then they'll perform the repairs, and then hand you a fat bill for the work before you get your keys back.

Ha yes arguing the Magnuson-Moss Warranty Act will be a pain in your rear. I bet that vehicle had a custom tune job that was burned into the PCM permanently. I've ask two different dealerships here and they don't care about the "canned tune" jobs, just the completely custom tunes. Again, everyone has to understand that you "pay to play" no matter how you look at it.

We can both agree that GM's calculation of MPG on the DIC has always been very optimistic. I have never trusted anything other than hand calculations when it comes to fuel mileage.