LM7 Tahoe - increased fuel consumption

[Askold_JB]

Hello there!
My first post for this forum, so excuse me plz If I will now start asking silly questions.

Get my Tahoe LT 4WD 2006 this June.
Solved a lot of problems already (thanks to this forum too!) and still happy to drive this big SUV.

Right now I really need an advise from experienced users.
Once I bought my Tahoe the gas mileage was just perfect- I did 20-21 (11ltr/100km) on highway and around 15 in city.
Car was at stock 17" 265/70 tires and had 120k miles.
Does it sound real, btw?

Then from month to month my mpg began to deteriorate SMOOTHLY.
Now it is around 14 mpg on highway (WTF!).
Month ago I installed 33" MT tires, but that is not the main reason 'cause mpg was ****** already even with stock tires.

So this is the problem.
Here is the car status:
1) Plugs are new or like new (ACDelco) and OK
2) All ignition coils seems to be OK
3) All injectors have been serviced, ultra-sonic cleaned and OK
4) All transmission liquids, engine oil, all filters are new or just like new.
5) Rear end, transfer case, AT - OK
6) Throttle body was cleaned, checked and OK
7) Engine runs smoothly, but sometime has misfire at IDLE (chaotic ones, misfire jumps from cylinder 1 to 8). Misfire problem actually appears after the moment I already noticed increased fuel consumption. So it maybe connected or it is a another different problem ))
9) FUEL pressure - never checked, but fuel pump was replaced 30k miles ago for a new genuine.
10) LEAN codes - none.
11) OBD data looks okay, I can not see something wrong, but I suspect MAF maybe is not OK.
MAF lean code appeared once few weeks after I bought the car. I replaced air filter and cleaned MAF with MAF cleaner. LEAN code gone away, OBD data looks great.

The question is - what should I check more or what should I try to replace to get the MPG I had before.

Thanks in advance!

 

[Danny3737]

It would be easier to diagnose your problem if you had not already changed your tires. Did you change your wheel size as well? Did you lift the vehicle as well to fit those tires. Regardless, those 33” tires with an aggressive tread pattern aren’t doing you any favors. What size motor do you have? I’m guessing you have 4WD. If you were getting 20/21 before, you must not have had much traffic and didn’t drive over 60 MPH. One other thing to consider is after you changed your tire size ( wheels?) Your odometer is probably off. Did your have your vehicle realigned after the tire swap?

 

[iamdub]

All considered, and off the top of my head, I'd investigate the catalytic converters and oxygen sensors (the "upstream"/pre-cat ones).

 

[swathdiver]

All considered, and off the top of my head, I'd investigate the catalytic converters and oxygen sensors (the "upstream"/pre-cat ones).

That would be my first look, graph them sensors and watch the temps and the LTFTs.

 

[Askold_JB]

It would be easier to diagnose your problem if you had not already changed your tires. Did you change your wheel size as well? Did you lift the vehicle as well to fit those tires. Regardless, those 33” tires with an aggressive tread pattern aren’t doing you any favors. What size motor do you have? I’m guessing you have 4WD. If you were getting 20/21 before, you must not have had much traffic and didn’t drive over 60 MPH. One other thing to consider is after you changed your tire size ( wheels?) Your odometer is probably off. Did your have your vehicle realigned after the tire swap?

As I mentioned before - bad mileage was already there even with stock tires.
Regarding to the lift - just changed rear from LT Autoride suspension to Z71 (shocks and coils), and aftermarket torsion keys on the front just to keep the front height at the factory defaults (old keys were almost fully engaged, especially at the driver side).

LM7 - is a 5.3l, and yes it 4WD. Drivetrain is 3.42 btw
Wheels were realigned properly not a long time ago after changing tires.

About my driving style - yes, I could say - granny would be proud )

 

[Askold_JB]

All considered, and off the top of my head, I'd investigate the catalytic converters and oxygen sensors (the "upstream"/pre-cat ones).

That would be my first look, graph them sensors and watch the temps and the LTFTs.

Thank you guys! Will try to figure it out, I will get some graphs, hope smb can help me with reading it correct.

 

[Askold_JB]

Just get recorded O2 data and STFT+LTFT + MAF in 2 situations:
1) idle, warmed
2) driving highway around 50 mph with stable throttle.
Please find it here, files too big to be attached. https://photos.app.goo.gl/GBfWSemN3AEWKHcg8

swathdiver
What "temps" did you mean? I only see coolant and AT temps available? Should be there more?

 

[afpj]

It's a little hard to tell with that busy graph, but it seems like the pre-cat O2 sensors are a little lazy.