2012 Yukon Denali 6.2. Poor gas mileage, no codes thrown and no check engine light

[Gearz]

Oxygen sensors are only good for around 100K. My Tahoe LTZ 5.3L was having lack of power getting on the highway like starving for fuel so I installed 2 new pre cat sensors and the engine came back alive and BTW it had no codes just lazy sensors. Your fuel smell from the exhaust could be a bad purge valve sucking fuel threw the canister plus you can try cleaning the injectors and if all else fails do a exhaust back pressure test to see if the cats are plugged. Of Coarse the normal things like plugs fuel pressure MAF sensor for dirt and intake gaskets etc.....

 

[dkad260]

I don't understand why the computer would display the AVG mpg declining while the truck is sitting still?

With what's already been mentioned.think about this..

If you filled your tank, drove 2 miles home then idled the tank dry, your final MPG would likely be the lowest reading of .1 MPG.

That's the somewhat "Pandora's Box" of seeing your average MPG. Ask your wife if she had the Average MPG on a different car.

The 14 MPG city is an average.
The 19 MPG HWY is an average.

On the highway, you will be in the single digits pulling a hill, then be at 70-99 MPG on a coast downhill.

Here's the key point...

If you just filled up sit at a light, and just reset your AVG MPG, your AVG MPG will decrease and change quickly because you are averaging small recent inputs.

BUT....after you drive half the tank down, and you have many, many more "plot points", the AVG MPG will change very slowly....more data to average.

If I fill up and sit in traffic before the highway, my MPG is absolutely between 8 and 10. But when I hit the road, it's slowly climbing into the high teens.

Also, using 93 will net you more MPG, you can pretty much take that to the bank.

I lose between 1 and 2 MPG over the life of the tank if I used 87. Only did that a couple times and stopped to lessen any damage from detonation.

 

[donjetman]

This is a new symptom after replacing the new parts I mentioned. I don't understand why the computer would display the AVG mpg declining while the truck is sitting still?

x2 what @wjburken said. idling is a mpg killer.

 

[BG1988]

if bad yeah, I've read they can drift or get slow to react. before they hit a point they throw a code. they are considered a maintenance item even though most don't and never have an issue.


the adv fuel mileage is just math something like fuel injector opening time, rpm to mph. if your not moving its averaging the fuel use with zero milage added. it seems to happen much after after a fresh reset. if it hasn't been reset in months, you shouldn't see it move much.


other. guys might know more, but maybe there's a leak in the evp repairs that the ecm is seeing and adding more fuel.

rule of thumb I've seen is long term fuel trims +/-5% are good, of they are up around +15% there is something gone on.

i was seeing +13 to +15 LTFT barely getting 12.5-16mpg used royal purple fuel injector cleaner 20oz bottle FYI

now it's 25-26mpg v8 mode

fuel trim is in the +3 range now most likely being the air filter..

also it might be contaminating the oil with fuel if it has a HIGH fuel trim

 

[BG1988]

6.2s are very efficient engines for what they are. They burn about a gallon an hour idling. The 5.3 burns about .78 gallons per hour idling.

If you run through a tank of slow speed city driving, where your average speed was below 22 mph, she will have used fuel at a rate of 1.5 gallons per hour versus 1.3 GPH for a 5.3.

Using the data from your DIC during refueling you can calculate how the engine is performing against how it's being used.

The data in green comes from the gas pump and my DIC. Excel is then used to make automatic calculations some of which are shown to the right.
View attachment 406014

When we're driving in the city, the truck will go 12-15 hours before requiring fuel again. Out on the highway she can run for about 8 hours before requiring fuel while still having a healthy reserve.

More important, and tell your wife this, you know how many people have died in a full length Yukon XL 4x4 or AWD since 2009? None. Not since I checked last anyway with the IIHS.

We had a guy join this forum a few years back, he lived and worked in downtown Miami and was frustrated with his 6.2 Tahoe, it barely got 10 mpg on a tank of gas. We finally convinced him to go eat a hamburger in Belle

Gas station matters I have been to many gas stations Pilot fuel station (Truck gas station )has always netted the BEST MPG for me in both the SUV and Corolla (44MPG highway 1.8L engine 87 octane) the SUV 25-26 in V8 mode (27.5 with AFM) in the 6.0L tahoe.. remember transmission matters 100% GAS mode

If It had a new battery it would be 40-45MPG in the tahoe the same routes i take...... as i would take advantage of the EV mode

 

[BG1988]

I'll looking around in there tonight.

there's been a bit of debate over the years of the hybrid can use e85. all the marketing brochures list this Gen as flex, but the cap isn't yellow. I'd bet it has all the same fuel pumps and lines thou. in hp tuners there is alcohol tables and they are populated.

a e % reading would lead to yes, but I have meant to research injector sizing in these trucks, usually the flex has bigger injectors.


sorry to hi jack the thread, but yes I've seen a few threads about e% being off and needing reset in the trucks that are flex but don't have an actual sensor. something for the op to check forsure

do let us know the results of it..i would use E85 as it would be cheaper per mile at 18 cents instead of 0.215/mile that would save $3.40 per 100 miles 15$ a week saved

 

[Doubeleive]

it was 105 here today, i leave mine idling as much as possible, running in the store, picking up kids from school, dropping of packages, etc, etc
doesn't seem to affect my mpg at all, but I also do not drive like a normal person. it took about 2500 miles of mostly highway driving to get it up to 12.5 from 10 average.
if the wife drives it she gets about 13 around town
most of your mpg is directly related to the big pedal, presuming everything is working properly.