Decreased MPG 2015 Suburban

CTburban · January 19, 2018

I just passed 50k miles on my 2015 Suburban LT. I use it for around town driving and once a month for a road trip for work. I’ve had it for over 3 years now. For the majority of my ownership, around town was 16-17 MPG and my road trips once a month were 19-20ish. I was super pleased with my mileage and pay close attention to how the car is doing, even watching for the shift to 4 cylinders.

Over the last several months, my around town MPG dropped, but I figured my routes changed to include more steep hills more often and wrote it off. It went down to around 15-16 MPG on average. I also took a different car for my road trips once a month. However, over the course of the last month, my MPG have completely tanked. I called it quits when I got 14MPG on my road trip this month! I had the fuel injectors cleaned befor that trip, too.

I brought it to the dealership, but they didn’t have any codes and didn’t waste their time troubleshooting. They told me it was the cold weather and the gas and my driving with the heat on. They said nothing was wrong with my Suburban.

Now, this is the 3rd winter I’ve had my Suburban. I have gotten my gas at the same station all 3 years. I completely understand the increased ethanol in the winter months. However it never affected my MPG this much. I even tried a few different stations and paid more for gas than ever trying to eliminate the gas being the issue. I also understand the accessories adding a strain on my MPG.

If the car isn’t faulting anything, where should I start looking? I have read all the ideas about O2 sensors and spark plugs and everything. A sales associate at the dealer who knows my technical obsession also mentioned they have a nifty box they put in the passenger seat to detect vibrations for cylinders misfiring and spark plugs and other possible options instead of the old way of shutting down cylinders one by one. Where do I start?

The Zip · January 20, 2018

Don't think anything is wrong with your truck, that's why you didn't get any codes. But there are some other factors involved with mpg. In the winter months the tires loose more pressure than in the summer. Grossly under inflated tires will have an big impact on mpg. Headwind is also an uncontrollable factor in a vehicle that size. How well the truck breathes determines how hard the engine works which can cause a loss in mpg.

Do this first. If you can (based on road conditions), run your tire pressure at 32 psi hot and change the air filter. Drive your normal routes and see if this helps. If you start with the O2 sensor, plugs, and a nifty box, you may get a self-inflicted "code" you don't want to see.

How, why, and where did you have the injectors cleaned?

You got extremely oversized tires or something external added that may be catching the wind. What do you mean by accessories adding a strain on mpg?

CTburban · January 20, 2018

My tires are standard size for the vehicle and I’m constantly putting air in them as it gets colder. I had the dealer do last oil change/tire rotation/engine air filter and fuel injector cleaning. They recommended the fuel injector cleaning at 30k miles but I put it off until about 45k. Even at 45k it seemed a bit early, but I’d already started to experience my fuel consumption issues. So I figured I would have that done to try to help.

The “accessories” was their fancy way of saying this is also because I have the heater on in the cold. I don’t run it crazy high, and I don’t turn it on until the car is warm.

Today the weather warmed up significantly, and it still took an extremely long time for my engine temp to get to normal readings. I made it to my destination as the gauge finally read 210–my normal reading. I even let the car run to warm up off the cold marking before I left.

And I don’t want to self-induce a problem, but my power train warranty will be gone in the next 3 months. I would rather figure this problem out while I have a chance of having something covered.

Thanks!

swathdiver · January 20, 2018

Well you did say your route changed and so will your mileage. I was testing my mileage the other day and went from dry to wet and lost 1/2 mpg and then drove into a torrential storm and lost over a mpg more. While I have no experience, you fellas in them yankee winters also lose mileage because of the cold and the less efficient fuel. I would say they use less ethanol because winter blends of E85 contain less ethanol and more gas to aid in starting. Down here we have one kind of E85 and my truck stalls on remote starts on those 30s and 40s cold morning we've been having but is fine on the second start once the fuel from the tank makes it to the injectors. Anyhow, I would recommend that you get a fancy scan tool that can read all those engine parameters. The Tech2 doesn't work on your truck but the MDI with GDS2 software on a laptop is what the dealerships use. There are other ones that interface with the aftermarket navigation units, now that is cool too!

Tire air pressure, clean engine oil, clean fluids in your differentials, transfer case, transmission, etc. all contribute to increased fuel efficiency.

There's probably nothing wrong with your truck, just wait til spring and if you haven't done so already, start an accurate log and get something so you can see what the computer sees with regards to all the engine sensors. I love it looking at that stuff too.

Jmiller11331 · January 21, 2018

Cold weather will also kill gas mileage drastically too

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Ben Braswell · February 11, 2018

I have had very similar effects with my 2016 this winter. My truck has a 5.3 with the Max Tow Pkg. I wrestled with the dealer as well over this and got mush for causes... I have come to conclusion that this is caused by temps. Whenever temps are in the teens and so, I get lousy mpg especially on short trips to and from work. The loss is a bit more than 10%, also get trans hunting, clunking and symptoms that worried me a lot! As soon as temps get into 40's and above the mpg returns to normal and other symptoms go away... I don't like it, but no codes and baffled dealer techs that treat you like your a problem make things worse. I know AFM gets me better mpg and best has been 32.2 over 25 mile avg, going down from mountain areas coasting. But I still wonder about longevity etc. My 2001 5.3, owned since new, was nearly bullet proof. It also seemed to be higher quality build. The 2016 had wiring harness damage that was re-spliced under warranty, and today I noticed water leaking in dripping from the ebrake pedal. Several post on this issue in the forum.

I like doing my own work/maintenance and really dislike going in to speak with people in service depts that couldn't change a brake pad and think mechanics is something owners have no knowledge of. Mostly it's insulting, but now i gotta go back and deal with em....again.

CTburban · February 16, 2018

So I took it back to the dealer again. I listed all my issues and got a service rep who took me seriously this time. I had to pay for the diagnostics, but I had a thermostat assembly that was bypassing/stuck open. That was under warranty and the change has completely fixed my issue! My engine was indeed working way too hard becaus it never thought it warmed up. No codes for that. It took a real mechanic to troubleshoot that down for me.

Good luck with your truck and dealer!

Tahoe02 · February 18, 2018

Good find by a real mechanic. I would buy him a case of beer. Others need to look at this thread if they experience the same issue. Thanks for reporting back to let us know what the final out come was.

Z45 · February 18, 2018

Had you been getting a coolant odor as well??? There is a TSB about over-cycling thermostat that also cause the radiator to leak (fatigue). The fix is a new stat housing+stat+radiator. Happened on my 2015 Sierra. You could see coolant staining on the radiator cores. Took several complaints to dealer to get fixed, although I never notice mpg issues but then again I never pay a lot of attention to mpg.

CTburban · February 19, 2018

Interesting you mention this. I found the letter they mailed about that the day after I had it fixed. I am still having the coolant smell after I feel any residual from the work should have burned off. I do think I’m going to have to bring it back to have it checked. How did you finally get the dealer to look for that?

I do think my thermostat was the main problem. Now I just need to coolant smell to stop. I’ve been away, so I’m going to have to see if it is still there when I get home.