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

[testdepth]

While sitting in the truck at idle I can watch the computer display "Average fuel mileage" it starts dropping while just sitting there. 1/4 tank of gas in it. The computer started at 8.2mpg which is low and in about 15 minutes the computer read 6.9 mpg. My wife is complaining that this truck is using too much gas. This truck has the 6.2 flex engine and normally around town its about 14mpg and thats what the display shows.

I own a very good active scan tool that allows me to check and activate most things on this truck.

No check engine light. No codes are shown. No misfires on any cylinder. I removed and cleaned the MAF sensor and the scan tool shows it functioning.

I do not smell any fuel while I'm under the truck and don't see any fuel leaking.

About 3 months ago I had to replace the Fuel pump, EVAP canister, EVAP Solenoid and purge valve.

Does something have to be reset so this system gets better gas mileage? ECU?

 

[j91z28d1]

if there's no miss, you might try replacing the o2 sensors. that's kinda the final check in the system to tell it to run richer than it should? well I guess dirty injectors could effect it some. what do the long term a short term fuel trims look like?

is this something new? like it got better mileage before? the change in Temps being a 100degs everywhere can effect a bit.. ac max load and heated fuel in the tanks mean you're getting less for the same flow metered amount.

but in the end, I have a feeling that 12 around town in the big 6.2 isn't that far outside the normal?

 

[testdepth]

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?

You are right about the weather being hotter now than before.

I assume if the O2 sensors are bad it would throw a code?

 

[wjburken]

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?

You are right about the weather being hotter now than before.

I assume if the O2 sensors are bad it would throw a code?

You trucks average fuel mileage will definitely drop while just sitting there. The computer is simply measuring the amount of fuel burned against the miles driven. When its idling, you are burning fuel but going no where.

 

[j91z28d1]

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.

 

[PG01]

Idling always drops fuel mileage/ range, on anything. I watch both my trucks do it, my Honda goes from 34ish to 27ish just sitting waiting to bolt outta work with a/c blowing. My company has anti idle policies for the trucks not only for environmental reason but gas savings….

 

[PG01]

Ps show your wife these posts so she understands it’s normal. Then get out on the highway at 65-70 and watch it go up together

 

[Doubeleive]

While sitting in the truck at idle I can watch the computer display "Average fuel mileage" it starts dropping while just sitting there. 1/4 tank of gas in it. The computer started at 8.2mpg which is low and in about 15 minutes the computer read 6.9 mpg. My wife is complaining that this truck is using too much gas. This truck has the 6.2 flex engine and normally around town its about 14mpg and thats what the display shows.

I own a very good active scan tool that allows me to check and activate most things on this truck.

No check engine light. No codes are shown. No misfires on any cylinder. I removed and cleaned the MAF sensor and the scan tool shows it functioning.

I do not smell any fuel while I'm under the truck and don't see any fuel leaking.

About 3 months ago I had to replace the Fuel pump, EVAP canister, EVAP Solenoid and purge valve.

Does something have to be reset so this system gets better gas mileage? ECU?

that's kinda low unless she drives like me which is highly doubtful.
1. how many miles on it?
I drive mine like a doberman is chasing me on a bike, so I average around 10mpg with dips down to 9.5 and on long drives around 12
a few years ago it had dropped to 8 and stayed there, in my case it was a bad cat after trying to figure it out for a while it decided to finally throw the code.
these are driving averages, I don't sit and watch it while idling but I would assume 8 while idling is still pretty low

 

[testdepth]

Ps show your wife these posts so she understands it’s normal. Then get out on the highway at 65-70 and watch it go up together

Well that didn't work.

There must be a reset or relearn for the MPG display because now it shows 6.4mpg.

 

[PG01]

Well that didn't work.

There must be a reset or relearn for the MPG display because now it shows 6.4mpg.

Leave it on that screen and press the check mark. Press and hold

 

[petethepug]

You can’t change the milage but you can change the driving behavior or the fuel. For me it’s easiest to run e85 vs changing the driving behavior.

$104.30 vs $180.09 for 91. Now that the fuel & evap system has been refreshed, the tank will hold a healthy 31.6 gal of fuel. Prior to that I could only get 26 gal in on a bone dry tank. The DIC accurately showed I had 15 miles of fuel left.

 

[B-train]

Will your scan tool reset the alcohol monitor? I wonder if it's stuck on a setting for e-85 and dumping fuel in.

 

[j91z28d1]

Will your scan tool reset the alcohol monitor? I wonder if it's stuck on a setting for e-85 and dumping fuel in.

I always forget about that with these trucks.

do you know what it's under on a tech 2? I'm curious to see if mine has a reading.

 

[j91z28d1]

You can’t change the milage but you can change the driving behavior or the fuel. For me it’s easiest to run e85 vs changing the driving behavior.

View attachment 406013

$104.30 vs $180.09 for 91. Now that the fuel & evap system has been refreshed, the tank will hold a healthy 31.6 gal of fuel. Prior to that I could only get 26 gal in on a bone dry tank. The DIC accurately showed I had 15 miles of fuel left.

thats interesting pricing. I stopped for fuel yesterday, 87 is 3.49 I think it was, 93 at 4$ and e85 was 3.00$

kinda. interesting e83 is about the same as you but the gas is much higher.

it feels like a scam all the time haha.

 

[Fless]

I always forget about that with these trucks.

do you know what it's under on a tech 2? I'm curious to see if mine has a reading.

On the menus for my '04 the alcohol content is under the Engine Data 2 menu.

 

[j91z28d1]

On the menus for my '04 the alcohol content is under the Engine Data 2 menu.

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

 

[swathdiver]

While sitting in the truck at idle I can watch the computer display "Average fuel mileage" it starts dropping while just sitting there. 1/4 tank of gas in it. The computer started at 8.2mpg which is low and in about 15 minutes the computer read 6.9 mpg. My wife is complaining that this truck is using too much gas. This truck has the 6.2 flex engine and normally around town its about 14mpg and thats what the display shows.

I own a very good active scan tool that allows me to check and activate most things on this truck.

No check engine light. No codes are shown. No misfires on any cylinder. I removed and cleaned the MAF sensor and the scan tool shows it functioning.

I do not smell any fuel while I'm under the truck and don't see any fuel leaking.

About 3 months ago I had to replace the Fuel pump, EVAP canister, EVAP Solenoid and purge valve.

Does something have to be reset so this system gets better gas mileage? ECU?

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.

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 Glade, 85 miles away, and he quit complaining about it.

 

[swathdiver]

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

Because it is. As time goes on, the average speed decreases and therefore the fuel efficiency does as well. It's a math equation or should I say, multiple equations.

On a GMT900, the alcohol content is found under the "Fuel Trims" menu.

 

[vcode]

Because it is. As time goes on, the average speed decreases and therefore the fuel efficiency does as well. It's a math equation or should I say, multiple equations.

On a GMT900, the alcohol content is found under the "Fuel Trims" menu.

Yep, reset it while you are idling and it will read 0.0mpg until you are moving!

 

[Chris Mosher]

Smell your oil, does it smell like gas? If so, your engine doesn’t like idling very long. I replaced O2’s, plugs/wires, MAF and PCV’s and problem went away.