Poor Fuel Mileage
Poor Fuel Mileage Have owned a 2002 1500 with 5.3 LM7 for years now. Over the past year the fuel mileage as gone down. I use to easily average 16-17 on the highway, and maybe 14-15 when off road. Now it seems no matter what I'm averaging 13mpg. I can't figure out what is the cause of poor fuel mileage. I have checked logs from years ago versus now, and everything looks good. I changed air filer, fuel filter, o2's, plugs, wires, and swapped coils, with another engine, with no change. I checked compression and all cylinders were all around 185psi.
Not sure what is causing the worse fuel mileage. I did have a u-joint go bad, but i fixed it and still getting bad mileage. Any ideas or suggestions on something to log to see if it is the cause?
My next idea was to possibly tune out the MAF and O2's for SD tune, and see if that helps any.
Not sure what is causing the worse fuel mileage. I did have a u-joint go bad, but i fixed it and still getting bad mileage. Any ideas or suggestions on something to log to see if it is the cause?
My next idea was to possibly tune out the MAF and O2's for SD tune, and see if that helps any.
Um the cats might have fallen off years ago.....
Actually I did get new tires. I went to a Nexen 10 ply tire cause it was cheaper then the old Goodyear Wranglers. (had too many sidewall punctures with the wranglers).
I track my fuel mileage every tank, and graphed the mileage over the past few years. The drop didn't suddenly occur when the tires were put on. But slowly decreased around the time of the new tires. So i wasn't convinced the heavier tires caused 20% fuel mileage drop.
I might have to swap wheels and tires with my dads pickup to see if there is a change.
Actually I did get new tires. I went to a Nexen 10 ply tire cause it was cheaper then the old Goodyear Wranglers. (had too many sidewall punctures with the wranglers).
I track my fuel mileage every tank, and graphed the mileage over the past few years. The drop didn't suddenly occur when the tires were put on. But slowly decreased around the time of the new tires. So i wasn't convinced the heavier tires caused 20% fuel mileage drop.
I might have to swap wheels and tires with my dads pickup to see if there is a change.
O2 sensors? My guess is they are wore out, and are either not swinging rich/lean as fast as they should or not working properly at all. You do not have to get a code, for this to affect mpg's. MAF may also be dirty, playing a part in this.
Since it sounds like you have HPT or EFI, have you looked/logged the O2's to see how they are swinging?
Since it sounds like you have HPT or EFI, have you looked/logged the O2's to see how they are swinging?
TECH Enthusiast
Joined: Jul 2010
Posts: 593
Likes: 29
From: Florida
That 13mpg might be the combined city and highway. top off the tire pressure first thing in the morning before moving the truck. than ride on the highway at 70mph then reset the mileage to O to get the true highway reading. ( 20 minutes )
I cleaned the MAF (sprayed down the wires with brake clean, not sure how else to clean it), and have also installed brand new o2's since the old ones had approximately 60,000 miles. No change in mileage. Looking at old logs and new logs the LTFT's were about the same anyway but thought i would replace them for piece of mind.
I have air tires up and down. put them to 50psi to see if it would help to no avail.
I have air tires up and down. put them to 50psi to see if it would help to no avail.
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Teching In
Joined: Oct 2017
Posts: 17
Likes: 0
From: ID
They make MAF specific cleaners, not sure if brake clean is good for them.
Also this time of year we are on the winter blend fuel, typically worse MPG from that, which may be compounding some other problem you have.
How are your brakes? Maybe one of them sticking a bit?
Also this time of year we are on the winter blend fuel, typically worse MPG from that, which may be compounding some other problem you have.
How are your brakes? Maybe one of them sticking a bit?
i know they make maf cleaners but didn't have any. it didn't really look dirty but sprayed brake lean on it to make sure it was fine. seems like it is reading normal. thus i was thinking about changing the tune so it doesn't use the maf. maybe that would restore fuel mileage.
however if the maf is reading poorly and it has new o2's you would think you would see high correction on the LTFT, correct? I'd assume the LTFT have final say in how the engine runs?
however if the maf is reading poorly and it has new o2's you would think you would see high correction on the LTFT, correct? I'd assume the LTFT have final say in how the engine runs?
I was thinking maybe after 160k miles it is time to pull the heads, shave them, cut the valve seats and install new seals. this should bump up power and improve fuel mileage. Wasn't sure if i could shave the 706 heads enough to get it up to around 10:1, since the lm7's are only 9.5:1 factory. I also live at higher elevation, so higher compression would be nice. Plus gain a little power.
Check your fuel trims. We see alot of older maf's start leaning vehicles out and the customer never notices until fuel mileage drop of loss of power towing. So far none have thrown maf codes when doing this either. They all had really high fuel trims.
What is considered really high fuel trims? looking at the logs I do see where it hit 7-8% occasionally.
Remind me, is a positive number on LTFT's mean it is lean and adding fuel?
Also if the fuel trims were high, doesn't that mean the o2's were doing their job. Essentially it is running stoich, its just that the O2's are relied upon more to make it run at 14.7 afr instead of the maf?
Remind me, is a positive number on LTFT's mean it is lean and adding fuel?
Also if the fuel trims were high, doesn't that mean the o2's were doing their job. Essentially it is running stoich, its just that the O2's are relied upon more to make it run at 14.7 afr instead of the maf?
On my camaro when i went boosted, I switched to a COS and turned the MAF off. I'm wondering can i perform the same steps to turn the MAF off with out a COS?
BTW, I'm using efiLive. I was thinking to test if the maf is the cause I could simply make the changes to the existing tune to turn the maf off. If fuel mileage increase then I know the maf was at fault. will this work?
BTW, I'm using efiLive. I was thinking to test if the maf is the cause I could simply make the changes to the existing tune to turn the maf off. If fuel mileage increase then I know the maf was at fault. will this work?
TECH Senior Member
Joined: Jun 2016
Posts: 22,291
Likes: 3,616
From: Central Cal.
IF the MAF is operating correctly, you will get better mileage than with a speed-density tune. MAF enables more precise AFR calibration. Speed-density is calibration by educated guess/estimation. MAF is actual air measurement.
I would address the reason why it's using more fuel before making actual changes. LSX Power Tuning made a good point. Make sure the MAF is doing its job before counting it out.
I would address the reason why it's using more fuel before making actual changes. LSX Power Tuning made a good point. Make sure the MAF is doing its job before counting it out.
How do you know if the maf is working properly? Simply replace it? As how do i know what air flow I'm logging is correct?
I wanted to turn off the maf in my tune, simply to see if it would run better or worse. I just wasn't sure if I needed a COS to do so, or if i can make the changes in the current OS it is running. try it out, and if it seems to run better and get better fuel mileage then I can replace the MAF.
I wanted to turn off the maf in my tune, simply to see if it would run better or worse. I just wasn't sure if I needed a COS to do so, or if i can make the changes in the current OS it is running. try it out, and if it seems to run better and get better fuel mileage then I can replace the MAF.
Teching In
Joined: Jul 2018
Posts: 31
Likes: 0
From: SoCal
I typically see positive fuel trims at low air flow and negative fuel trims at higher air flow if the MAF is dirty...
I made a special brush to clean the MAF sensor elements, it is shaped like a "J" hook. Another thing to think about is cam timing. Has the timing chain retarded the cam in the last few years?
Good luck.
I made a special brush to clean the MAF sensor elements, it is shaped like a "J" hook. Another thing to think about is cam timing. Has the timing chain retarded the cam in the last few years?
Good luck.