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poor gas mileage!

This is a discussion on poor gas mileage! within the GM Trucks forums, part of the Vehicle Specific category; I've had this 03 silverado 5.3, 82,000 mi. for a little over a year. I only get a little over ...

  1. #1
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    navy blue
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    poor gas mileage!

    I've had this 03 silverado 5.3, 82,000 mi. for a little over a year. I only get a little over 300 mpg from the 26 gal. 12-13mpg. Since I've had it- new plugs, wires, pcv, cleaned tb (TBW - no egr), cleaned MAF, fuel filter, ran injector cleaners in tank,and CAI. should I replace some 02's? How many are on it? 4? Are they all the same? I was on rockauto, and all the o2's listed say they can be used for up and downstream. Any input I would really appreciate. Thanks guys!

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    navy blue
    1998 camaro ss

    Oh by the way it runs great! No misses, no codes (well no engine light). Plenty of power

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    Senior Member 5.0THIS's Avatar
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    If it's running right, I wouldn't go throwing O2 sensors at it yet. That's not a cheap proposition. Dumb question... but have you put a new air filter in it? Also, fuel injector cleaners that you put in the fuel tank are only going to be so effective. GM has a top end cleaner that gets run directly through the fuel rails, and is highly concentrated (fuel pump fuse is pulled, and an air pressure operated canister has fuel and the cleaner mixed and hooked up directly to the fuel rail). That being said, if the truck runs and emissions as it should, then the problem may be with your right foot.

    What configuration is your truck? What wheels and tires? What kind of driving? All of those can effect mileage substantially.

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    Quote Originally Posted by 5.0THIS View Post
    If it's running right, I wouldn't go throwing O2 sensors at it yet. That's not a cheap proposition. Dumb question... but have you put a new air filter in it? Also, fuel injector cleaners that you put in the fuel tank are only going to be so effective. GM has a top end cleaner that gets run directly through the fuel rails, and is highly concentrated (fuel pump fuse is pulled, and an air pressure operated canister has fuel and the cleaner mixed and hooked up directly to the fuel rail). That being said, if the truck runs and emissions as it should, then the problem may be with your right foot.

    What configuration is your truck? What wheels and tires? What kind of driving? All of those can effect mileage substantially.
    The sensors list for $25 a piece for densos on rock auto. Thought that was pretty cheap. For the mileage, it's not a bad idea to replace them anyways. Just wasn't sure if I was gonna do all 4 or just the 2 upstream, but I think they all might be the same. I do mostly city type driving and don't really beat on my truck. I installed a leveling kit, but still have factory size hankook dynapros with factory style wheels. The air filter is a k&n and not very dirty.

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    Senior Member 5.0THIS's Avatar
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    Leveling kit+ city driving. But if you just want to throw parts at it, have fun.

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    Moderator Firebirdjones's Avatar
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    Doesn't sound like you've installed anything out of the ordinary that might kill the gas mileage.

    02's do get lazy over time. Most tuners prefer to change them about every 40-50,000 miles. Old sensors tend to switch slow so they don't meter the fuel as accurately. If that gets bad enough it will generally set off a slow switching code. With 80,000 miles and only costing $25 a piece it's cheap maintanance, I'd replace them if it were mine. Only need to change the front pair (before cats) as those are the only 2 that trim the fuel. The back 02's are just there to check cat efficiency.

    Other than that sounds like you've tried alot of other things. Keep the tire pressures up and be aware of your right foot, no reason why the little 5.3's shouldn't get 16-17 mpg around town. My 6.0 did.

  7. #7
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    navy blue
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    Exactly man I don't hear anybody really getting anything less than 15 or so. I guess I'll try the front 02's. All 4 of them are the same part # I think. As I seen on rockautos listings. Would of thought the ft. 2 would be heated and more $, but their all the same price up and down. Idk?

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    Moderator Cutlass's Avatar
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    I have an 2001 Silverado, extended cab, short box, 4x4, 5.3 V8 with 3.73 gears. I get 14 MPG in the summer and 13 MPG in the winter..mixed driving with a fair amount of highway. I think your in the ballpark. If you wanna swap out O2s, only the front ones could potentially make a difference in MPG.
    FBJ's 6.0 is a swap into an older vehicle with a tune. guys around here get 11 or 12mpg out of 6.0s from back then and 5.3 get 13 to 15 mpg. things like 2wd or gear ratios or different cab/box configs or tires can affect MPG. Leveling kit affects MPG too. the truck is more aerodynamic with the nose down. Its a brick with the nose up.

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    Moderator Firebirdjones's Avatar
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    Just to give you more of an idea that Cutlass touched on.

    It's a 72 blazer 4x4 with 33" tires, sits up in the air and shaped like a brick. I run 3.73 gears in it. 6.0 is a bone stocker with a tune, and a beefed 4l60E behind it that also turns a cast iron heavy NP205 transfer case. The truck scaled at 5,000 lbs. without anyone in it. Definately not a truck you would expect gas mileage from.

    16-17 mpg with strictly around town driving at 45mph or less. Long highway cruises I've gotten a best of 22 mpg, but will get 20 mpg regularly.

    The tune is a big part of it. I dialed it in with a wideband, and stayed with an SD tune because I liked the accuracy and throttle response (MAF still in place though for IAT temps but non functional as a fuel trimmer).

    So I see no reason for 5.3's to suck down 13 mpg in a sleeker newer truck, especially a 2wd. If the mechanicals are running top notch then I'd be tuning that thing asap.
    Last edited by Firebirdjones; 02-02-2014 at 07:30 AM.

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    Moderator Cutlass's Avatar
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    1999 Formula WS6 M6-sold
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    These Silverados aren't light weights either. there are about the same as your 72 Blazer if not heavier. It really depends on the cab/bed config and of course 2WD vs. 4WD. Start adding nerf step bars, bedliners, toolbox, and brush guards to a crewcab or extended cab 4x4... you're easily over 5500 pounds. 5.3s don't like to push around a lot of weight.
    A SD, no O2 feedback tune like yours can definitely get those numbers way up there. Of course FBJ's old man driving habits help too.

  11. #11
    Moderator Firebirdjones's Avatar
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    Haha, yeah old man driving habbits

    I agree the 5.3 will have to work harder to push the weight around. It's a good example of smaller is not always better when it comes to gas mileage.

    My father has a 2002 3/4 ton with the 6.0. It's a 4x4 extended cab, 33" tires added and suspension raised about 3" in the front for a level appearance. Nurf bars (or us old people call them steps ) and a spray in bedliner. Otherwise a stock truck. He reports a best of 14 mpg but gets 13 regularly with a mix of highway and city. I think that's about average. We are in the process of moving him out here. When that's done I will be tuning this truck with HPtuner. I'm curious to see what kind of MPG I can pull out of it while bumping the power. I know my father can't wait, he's complained of the mileage since he bought the truck brand new.

    For giggles, my 79 1 ton long bed pickup is a 2WD, 502 crate engine, 400 turbo (no overdrive), and 3.73 gears with stock size 9.5x16.5 tires (about 30" tall) I get 14 mpg highway and 11-12 mpg city. This truck is of course shaped like a brick and it scaled 5470 lbs without a driver. Really not all that bad on gas considering what it is.

    My 2005 Duramax 3/4 ton is better than my old truck on fuel mileage in comparison. Crew cab 4x4, Idaho Rob tune, intake and exhaust mods, 33" tires, raised in the front about 3" for a level appearance, nurf bars (steps ) and bed liner. It scaled at exactly 7,000 lbs without a driver. I routinely get 16-16.5 mpg around town and have gotten a best of 19 mpg highway.
    Compared to my 6.0 in the blazer they are about even on fuel mileage, but 2 completely different animals.

  12. #12
    Moderator Cutlass's Avatar
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    I don't know why I said nurf bars. I never use that term usually. I almost always call them tube steps.
    those 33" tires might give some advantage in making your 3.73 feel like a 3.42 or 3.50 gear in comparison to a 31" stock tire.
    OP, these trucks like to break exhaust manifold bolts. A leaky exhaust manifold might throw off your O2 sensor feedback and give you poor fuel mileage

  13. #13
    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by Cutlass View Post
    I don't know why I said nurf bars. I never use that term usually. I almost always call them tube steps.
    Ha, I knew what you meant. I've heard the term nurf bars for years, actually didn't think anything of it, other than some of us needing them to get in a truck,

    I wouldn't put them on my blazer and frankly, I was sick of climbing in and out of it 6-7 times a day. Even my wife and 14 year old son complained. And that was a stock height 4x4 truck, lol.

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    Quote Originally Posted by Cutlass View Post
    These Silverados aren't light weights either. there are about the same as your 72 Blazer if not heavier. It really depends on the cab/bed config and of course 2WD vs. 4WD. Start adding nerf step bars, bedliners, toolbox, and brush guards to a crewcab or extended cab 4x4... you're easily over 5500 pounds. 5.3s don't like to push around a lot of weight.
    A SD, no O2 feedback tune like yours can definitely get those numbers way up there. Of course FBJ's old man driving habits help too.
    I have a 4x4,standard cab, short bed, no accesories besides those step bars and tonneau cover. Nothing else. Never haul anything. I guess or you would think it would be one of the lighter versions of a silverado, but idk. Maybe I'll hit up a scale somewhere. I was gonna change the 02's,but was concerned about a tune also. I actually emailed frost and he pretty much said my mileage would be worse if I got a mail order tune, but I also told him I was adding headers too.since there's already a flow master catback on it. I honestly don't know what speed density tuning is or how it works. Would you say I would get better mileage if I got that kind of tune opposed to your basic mail order tune?

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    Moderator Firebirdjones's Avatar
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    Not sure how your mileage could get worse than the 12-13 you're currently getting I've never experienced worse mileage with a tune,,,speed density or not. Matter of fact after Frost sent me a revamped MAF table that I was using I actually picked up mileage in that car. Frost also explained it to me that with an MAF table dialed in correctly with a tune, it's not hunting around too much for the AFR and therefore it picks up better MPG, that's why I found better MPG when he revamped my MAF table.


    Some other things in these trucks (I know the 6.0's do anyway) is that they enter open loop under throttle much earlier in the tune than the passenger cars do and as a result revert back to WOT AFR's (enters power enrichment) that are much richer (stock is about 11.4 AFR). Just leaning out WOT AFR's not only picks up power, but less fuel is used, so if you are tipping into PE quite a bit, well then.... I dialed back the throttle angle for PE in my truck tune, so I could squirt around town in stop and go traffic under light to moderate loads without entering PE.

    I'm guessing GM does this early power enrichment on the truck tunes because these things are destined for towing and hauling stuff (it's a truck ) So they tune on the safe side of things knowing there will be heavy loads and alot of throttle tip in. Things like this kill MPG in daily driving though because I found while logging my truck it would enter power enrichment with very little throttle angle, even when mildly accelerating from a stop light, up a slight grade for instance.

    Another one is enrichment to cool the cats (basically dumping fuel) and all the LS programs I've seen has this. Not good for MPG. But you need to rid the truck of cats before eliminating this feature.


    Better timing tables, better VE tables, all create crisper throttle response and should take less throttle angle to accelerate up to speed = better mpg.

    These are my experiences, but Frost has been doing it for years, and he's a stand up guy to not want to sell you something that he says likely won't do what you're asking.
    Last edited by Firebirdjones; 02-04-2014 at 12:15 PM.

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    I'm gonna have to think about what to do with the tune after I install these 02's. If I get better mileage, great, if not, I'll probly get a tune, been wanting to get one anyway regardless of gas mileage. Would I have to tell him a SD tune? Or just basic tune? I'm not that familiar with some of the things you mentioned in last post regarding tunes Fbird. Thank you for the info though. I also would think I would get better MPG after a tune. As long as I'm light on the throttle. (Which I am most days)

  17. #17
    Moderator Firebirdjones's Avatar
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    I wouldn't be too anxious for a tune just yet. I'd still want to investigate why the truck is doing poor on mileage.

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    Quote Originally Posted by Firebirdjones View Post
    I wouldn't be too anxious for a tune just yet. I'd still want to investigate why the truck is doing poor on mileage.
    Indeed. See what I got when these o2's arrive. Got sensors! Hope their good enough. Didn't want the bosch

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    Moderator Cutlass's Avatar
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    Quote Originally Posted by Supersport0126 View Post
    I have a 4x4,standard cab, short bed, no accesories besides those step bars and tonneau cover. Nothing else. Never haul anything. I guess or you would think it would be one of the lighter versions of a silverado, but idk. Maybe I'll hit up a scale somewhere.
    Yes that would be a lighter one, probably well under 5000 pounds unless the frame is filled with concrete, hehe. IDK, I searched around the net a bit for 5.3 Silverado MPG and there's a lot of guys out there getting 12-14 MPG city.
    -------------------------------------
    Kooks 1 7/8" race headers, Kooks ORY, Borla catback, Nitto 555R, LS6 intake manifold, Shaner S3 ported stock throttle body, SLP lid, smooth bellows, JAAM Ram Air kit, Elite Engineering catch can, LS6 valley cover, EGR and AIR deleted, Frost tune, and Simpson Racing child car seat in the back. 13.2 @ 108MPH

  20. #20
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    I have the same truck 99 silverado ext. cab LT auto 5.3 4 wheel drive. I get 15 mpg in town. It has 133000 miles. I put duals and a c a i. I had a 1996 ext cab chevy with the 350 auto and that thing sucked the fuel. id be lucky to get 12 mpg plus the turning radius was huge . I was glad to sell it.

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