Suggestions for steady drop in MPG?

Mike534x

Well-Known Member
Hello everyone!

I've been having an issue with my Avalanche for the past year or so. When I purchased it originally with 79k in 2013, I would average roughly 14-15 around town/city driving and 18-19 on the highway. As of late, within the last 2-3 years the fuel economy has tanked so I average around 10 to 11 MPG...with summer blend and roughly the same maybe 1-2 MPG less with winter blend. Highway driving is roughly the same, probably about 17.5-18 give or take. Not sure if this matters, but I did keep it in storage during the last two winters, and brought it back out for the remainder of the year to drive around when doing work on the Envoy.

Yeah its a brick so I should not expect stellar MPG, but the drop seems to be big enough to look into. I'm not too lead footed either with it so I'm trying to see where a good place to start would be.

Thanks!

What I've done so far;

Tune up with AC Delco Iridiums at 89k
Swapped the wires 2 months ago when I started to have a rough idle which immediately cleared up
Cleaned the MAF Sensor. I was going to clean the throttle body but it looked fairly clean without much build up.
Replaced the Coolant Temp Sensor when I did the water pump/belts.
Ran a tank with two bottles of the Techron concentrate plus. If BG44k is worth buying, I'll try throwing that into the tank.
No engine lights/codes


Specs:

05 Avalanche Z71 (1500) currently at 101,779k
5.3 Flex
3.42 gearing
4x4
 

Chickenhawk

Well-Known Member
How does it perform on the highway? Does it seem to accelerate strongly or does it seem slower to accelerate? It could be your cat is starting to get plugged. (Running with a bad misfire or long-term low coolant temp can do that.)

Other than that, all I can suggest is to have a look at the oxygen sensor readings with a good scan tool. Also, what percentage of ethanol is in your local gasoline? This can easily reduce city mileage that much if they suddenly introduce 10% ethanol.
 

littleblazer

Gold Supporter
Hows your fuel trims?
 
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Mike534x

Mike534x

Well-Known Member
How does it perform on the highway? Does it seem to accelerate strongly or does it seem slower to accelerate? It could be your cat is starting to get plugged. (Running with a bad misfire or long-term low coolant temp can do that.)



Other than that, all I can suggest is to have a look at the oxygen sensor readings with a good scan tool. Also, what percentage of ethanol is in your local gasoline? This can easily reduce city mileage that much if they suddenly introduce 10% ethanol.
It drives great on the highway, the 3.42 gears don't really help much with acceleration but it has 0 hesitation once you get on it to make a pass. Before I changed the plugs it had no codes as far as a misfires go, but it did have a slight "miss" when idling that cleared up with the new plugs. The coolant temp sensor was still good, but I figured with it being a cheap part I saw no harm in replacing it while I did the water pump. I was wondering if the the 02 sensors could be getting old/slow, but it doesn't have any codes pending or stored. I can hook up torque and provide what its reading if that helps. All the stations around here us 10% ethanol, the nearest station without it is a good few hours away.


Hows your fuel trims?
Haven't had a chance to hook up Torque to take a look with being gone for the weekend, I'll post them soon!
 

littleblazer

Gold Supporter
It drives great on the highway, the 3.42 gears don't really help much with acceleration but it has 0 hesitation once you get on it to make a pass. Before I changed the plugs it had no codes as far as a misfires go, but it did have a slight "miss" when idling that cleared up with the new plugs. The coolant temp sensor was still good, but I figured with it being a cheap part I saw no harm in replacing it while I did the water pump. I was wondering if the the 02 sensors could be getting old/slow, but it doesn't have any codes pending or stored. I can hook up torque and provide what its reading if that helps. All the stations around here us 10% ethanol, the nearest station without it is a good few hours away.




Haven't had a chance to hook up Torque to take a look with being gone for the weekend, I'll post them soon!
If they're massively positive it may be time for intake gaskets. That made a huge difference on the 03.
 

Redbeard

Well-Known Member
As Chickenhawk mentioned check your o2 sensor. I have the 4.2 engine (2004) and when mine went out the milage dropped to what yours did to 10 mpg or so virtually overnight from a common 15-16 mpg running around in town. IIRC the sensors were replace 2 or 3 years ago making them last over 10 years, so as you mention they just might be getting "slow".
 
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Mike534x

Mike534x

Well-Known Member
Hey everyone, after a long delay I have the screenshots of the readings Torque gives me for the 02 sensors. I also took some of my Short and Long term fuel trim's as well.

First image is

Bank 1 Sensor 1 and Bank 1 Sensor 2

Bank 2 Sensor 1 and Bank 2 Sensor 2

Fuel Trim Bank 1 Long Term + Fuel Trim Bank 1 Sensor 1


Last one I mistakenly checked everything so its grouped Fuel Trim Bank 1 Short Term, Fuel Trim Bank 2 Long Term, Fuel trim Bank 2 Sensor 1 and Fuel Trim Bank 2 Short Term.
 

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