04 Sierra mileage question

[gdsqdcr]

Hey guys
Any insight would be useful. I have 04 GMC Sierra CC Z71 with the 5.3. I bought her 7-8 years ago with 50k miles and she currently has 175k. Been an exceptionally reliable truck.


When we first got it I would average mid 15's, low 16's in my commute. Now it is mid-high 14's. This drop has occurred and stayed consistently down for ~20k miles now. No leaks. Fuel system has been cleaned. Fuel filter is inside tank and is original. Tuneups are 100k mike interval. It occurred on time with all OEM parts. Exhaust, O2 etc are all original. No codes. No check engine light.

Any thoughts on to why mileage is dropping?

To be clear, this is a truck. A 4wd truck. I am not expecting great mileage, just wondering why the mileage is starting to drop.

[justbob]

Be lucky you get that.. My 04 Lariat w/160,000 gets 10.4! Your o2's are waaaay over due, that alone should make a difference.


Self proclaimed Builder Of Badassery.

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[Limited360]

I crack 14-15mpg with my Silverado SS...

I'd suggest new upstream O2's as a start if they are original at 175k miles.

Also has your state recently gone to the E10 standard?


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[Limited360]

California has gone to E10 for their new certification fuel since 2013MY, my guess is this may be more ethanol content in your fuel out there.


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[1Marauder]

Tires? Could be that after the first/second set... they changed brands etc?

[Marauderjack]

California has gone to E10 for their new certification fuel since 2013MY, my guess is this may be more ethanol content in your fuel out there.


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^^^+1^^^:cool:

[Limited360]

^^^+1^^^:cool:


Nerd talk :) Best part is arguing with OEM's when we do a cert test and have to charge them for NMOG analysis now due to the E10 standard in the test fuel... Funny how many still ignore this and test with indolene still...


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[gdsqdcr]

Tires have been same brand/size since 60k (took a mpg hit). They are BFG KO 265/60/18's.

10% ethanol mix ... I understand the fluctuation with the seasonal gas but had not considered the ethanol increase being a cause.


O2 sensors are original, past their useful life? Even with no codes? I can change those,it would not hurt anything.

[Limited360]

Tires have been same brand/size since 60k (took a mpg hit). They are BFG KO 265/60/18's.

10% ethanol mix ... I understand the fluctuation with the seasonal gas but had not considered the ethanol increase being a cause.


O2 sensors are original, past their useful life? Even with no codes? I can change those,it would not hurt anything.

O2's could be hanging on the switch from rich 2 lean a bit, the actual sensor my be coked up with soot, multiple variables. It may be functioning properly to not set a code, just not optimally. No promises on this however!

[gdsqdcr]

O2's could be hanging on the switch from rich 2 lean a bit, the actual sensor my be coked up with soot, multiple variables. It may be functioning properly to not set a code, just not optimally. No promises on this however!

Bought a new pair if up streams this morning. Soaking the old ones before I try and take then out right now.

Replacing them can't hurt, they do have a few miles on them .... :banana2:

[Limited360]

Let us know feedback!! :)


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[gdsqdcr]

Let us know feedback!! :)


Sent from my iPhone using Tapatalk so I may sound retarded.

New O2's are in. Initial feedback from the truck's mpg calculator suggest that it is getting better. Once I get a few hundred miles on her, I can make the final claim of 'yes' or 'no'.

Getting the old ones out, that was a challenge. They had settled in after a 175k miles and did not come out without a fight.

[dohc324ci]

That's some decent mpg I have a 2500 burb with the 6.0L and get 10-11mpg...

[gdsqdcr]

That's some decent mpg I have a 2500 burb with the 6.0L and get 10-11mpg...

I could not handle that MPG with the amount of miles I put on the truck. During winter/baseball season, we average 350 miles a week on the truck. Sometimes we hit 500 miles in a week. Our summer trips are 5-6000 miles. 10-11 mpg would be the death of my wallet.

I will also say, we run BFG KO in 265/65/18 tire size. That eats up mileage as well. =)

[Limited360]

New O2's are in. Initial feedback from the truck's mpg calculator suggest that it is getting better. Once I get a few hundred miles on her, I can make the final claim of 'yes' or 'no'.

Getting the old ones out, that was a challenge. They had settled in after a 175k miles and did not come out without a fight.

Hope the O2's help out in the long run!!! I need to change them out on my Silverado as well at some stage. 130k on it, original parts.. can not hurt to change.

I average 14.5 - 16 MPG in winter since I started calculating it by hand.. .I have a Silverado SS with 6.0L and 4.10 gears though... oh yeah 285 series tires don't help either.

[Curless]

Hey guys
Any insight would be useful. I have 04 GMC Sierra CC Z71 with the 5.3. I bought her 7-8 years ago with 50k miles and she currently has 175k. Been an exceptionally reliable truck.


When we first got it I would average mid 15's, low 16's in my commute. Now it is mid-high 14's. This drop has occurred and stayed consistently down for ~20k miles now. No leaks. Fuel system has been cleaned. Fuel filter is inside tank and is original. Tuneups are 100k mike interval. It occurred on time with all OEM parts. Exhaust, O2 etc are all original. No codes. No check engine light.

Any thoughts on to why mileage is dropping?

To be clear, this is a truck. A 4wd truck. I am not expecting great mileage, just wondering why the mileage is starting to drop.

It has been said but I will say it again...O2's, get OEM ones and have the MAF cleaned.

[03-300a]

I have a Yukon denali xl awd 6.0l with 26inch rims and I average 12.4 mpg

[gdsqdcr]

It has been said but I will say it again...O2's, get OEM ones and have the MAF cleaned.

Why OEM on the O2? I thought aftermarket would be okay in this area ... Course I bought Bosch and not OEM. Do I need to replace them ... I hope not.

As for MAF, cleaned yearly! It is part of my annual maintenance (25k miles) on the truck, along with transmission filter - axle fluid is changed every two years (50k miles).

[Limited360]

Why OEM on the O2? I thought aftermarket would be okay in this area ... Course I bought Bosch and not OEM. Do I need to replace them ... I hope not.



As for MAF, cleaned yearly! It is part of my annual maintenance (25k miles) on the truck, along with transmission filter - axle fluid is changed every two years (50k miles).


Bosch should be an ok replacement as that is what GM uses 99% of the time in these model years.

Idea of getting the OEM part is that the resistance places a huge factor in the fuel trims based on the signal...

Once again I wouldn't worry, I would bet the OEM part was a Bosch O2 anyways...


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[gdsqdcr]

Bosch should be an ok replacement as that is what GM uses 99% of the time in these model years.

Idea of getting the OEM part is that the resistance places a huge factor in the fuel trims based on the signal...

Once again I wouldn't worry, I would bet the OEM part was a Bosch O2 anyways...


Sent from my iPhone using Tapatalk so I may sound retarded.

Good! Thanks for the info. I think I got lucky ...

[clintons4]

My Silverado averages above 16mpg on the computer. 2005MY 4.8 2WD ex cab short bed. I have almost 200k original O2's, wonder if it is worth replacing them. One of the best vehicles I have owned. :banana2: