20% decline in MPG

[Chris Lieberman]

Ok 2017 4wd Suburban LT, 5.3.

 

In the 8000 miles that I have owned it, I averaged 16-18mpg depending on how much I used the remote start.

 

Now last time it was in the dealer for 3 weeks for a no start (theft module replaced according to Repair Order), I got it back and it seems they may have FINALLY fixed the no start issue. But the problem now is I am now getting between 11 and 14 which is a pretty sizable drop off. I can't see where what they did would have any affect on fuel mileage, but I am honestly not sure what could cause such a sudden large drop in mileage. Nothing else has changed. The only thing I have noticed myself is the voltmeter used to always show between 11.5 volts and 14.5 volts (on or off as I refer to it), but now I the voltmeter has not dropped below 14.2 since I have gotten the truck back which leads me to believe it's always on at this point. I have tested the battery several times and it's all good. 

 

Any ideas? Family of 8, I would love to get that MPG back to where it was money is always tight with that many kiddies.

 

 

Thanks in advance for any thoughts and ideas.

[Mandalorian]

I'd check the air filters, tires, brakes, fuel lines, etc the usual suspects. Could also be due to winter gas.

Edited April 30, 2019 by Mandalorian

[Chris Lieberman]

It maintained 16-18 all winter long, so I figured it might actually go up a little once it warmed up. Definitely didn't expect a decrease. Tires at 35-37 cold, air filter new. Doesn't seem to make any sense.

[Mikey999]

On 2019-04-30 at 8:48 AM, Chris Lieberman said:

It maintained 16-18 all winter long, so I figured it might actually go up a little once it warmed up. Definitely didn't expect a decrease. Tires at 35-37 cold, air filter new. Doesn't seem to make any sense.

When i switched over to snow tires on other rims my mileage  dropped.  I will see what happens when i switch back in may.

 

[Sierra Dan]

Is your AFM acting normal? How about engine noise? Are you hearing any ticking or noticing a lack or lag in performance?

[TXGREEK]

Just a thought, ethanol gas, sitting so long may have gunk up your fuel system? Try running a fuel system cleaner like “SeaFoam” and go from there.


Sent from above

[b1gted]

If I had to guess,  they flashed the ECM as part of the fix for the no start issue.  If there were any other updates needed to the ECM, they would do them at the same time.  It is possible that it changed some setting slightly resulting in less MPG.

 

Only a guess and my opinion.

 

[Chris Lieberman]

35 minutes ago, Sierra Dan said:

Is your AFM acting normal? How about engine noise? Are you hearing any ticking or noticing a lack or lag in performance?

I don't think think it's acting normal, but the dealer says no codes so it must be fine. It seems to take forever to drop into v4 mode (no joke over a mile of steady speed on a flat road before it drops right into V4 mode, then it seems to switch back to v8 mode for no apparent reason). I do hear a slight ticking sound, but I don't think it's out of the ordinary, and there doesn't seem to be any lack in performance. But it seems weird to me that the alternator never seems to switch off anymore. I have a radar detector that shows voltage and it never shows lower than 14.5 when it's running. When I first got the truck it would read 11.7 for what seemed like a long time, then you could see it start charging and mover up to 13.5 or so. Now it just stays constant at a high charge, but the battery tests fine.

[Chris Lieberman]

6 minutes ago, b1gted said:

If I had to guess,  they flashed the ECM as part of the fix for the no start issue.  If there were any other updates needed to the ECM, they would do them at the same time.  It is possible that it changed some setting slightly resulting in less MPG.

 

Only a guess and my opinion.

 

I also wondered that myself, it did seem to happen at exactly the same time.

[Chris Lieberman]

1 hour ago, TXGREEK said:

Just a thought, ethanol gas, sitting so long may have gunk up your fuel system? Try running a fuel system cleaner like “SeaFoam” and go from there.


Sent from above

I did actually order a couple cans of Seafoam, figured it can't hurt. Truck has 62,000 on it already.

[TXGREEK]

I did actually order a couple cans of Seafoam, figured it can't hurt. Truck has 62,000 on it already.

I run Lucas Fuel Treatment in every or every other full tank and have excellent results from doing so. I used to get rough idling in my 2016 5.3 Yukon but completely disappeared after a few tank full of the Lucas, 3 yrs later and still purrs like a kitten with excellent throttle response.


Sent from above

[KARNUT]

I have 5 vehicles ranging from 27-18-13-8-2 years old. These vehicles run mostly top tier gas. Never used fuel treatment, never a problem. I’ve seen people cure bad running vehicles with sea foam type fuel treatment. My experience if you use top tier fuel from a busy gas station, fuel problems would be rare.


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[TXGREEK]

I have 5 vehicles ranging from 27-18-13-8-2 years old. These vehicles run mostly top tier gas. Never used fuel treatment, never a problem. I’ve seen people cure bad running vehicles with sea foam type fuel treatment. My experience if you use top tier fuel from a busy gas station, fuel problems would be rare.


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I’d agree with you there if it wasn’t for my two separate issues I encountered with two separate Shell gas stations in the Houston area that I was pumping fuel for almost a year and never thought about what could possibly be causing injector issues in two separate vehicles till the mechanic asked what fuel you using? Fuel was not top tier, turned out to be the fuel. I ran Kroger fuel for about 1yr and never had any issues again especially since I add Lucas Furl Treatment in ever other tank, swear by that stuff. Never know where that fuel tank truck is coming in from.


Sent from above

[Tx_operator87]

I’d agree with you there if it wasn’t for my two separate issues I encountered with two separate Shell gas stations in the Houston area that I was pumping fuel for almost a year and never thought about what could possibly be causing injector issues in two separate vehicles till the mechanic asked what fuel you using? Fuel was not top tier, turned out to be the fuel. I ran Kroger fuel for about 1yr and never had any issues again especially since I add Lucas Furl Treatment in ever other tank, swear by that stuff. Never know where that fuel tank truck is coming in from.


Sent from above

I'm from the Houston area as well and in my years of driving, the only problems I've ever had with bad fuel was from a Shell station. I tend to stick to Kroger for the discount, HEB, or buccees. Have never had a problem with any of them or any injector problems

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[TXGREEK]

I'm from the Houston area as well and in my years of driving, the only problems I've ever had with bad fuel was from a Shell station. I tend to stick to Kroger for the discount, HEB, or buccees. Have never had a problem with any of them or any injector problems

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Isn’t that the truth, sad but true. I’ve seen the same fuel truck at a Valero and Shell and that’s why I run the Lucas Treatment, buy it in cases.


Sent from above