Drastic MPG drop

[GM Gearhead]

Anyone have issues with their 2015+ 5.3 liter dropping fuel mileage?  My 2015 K1500 suddenly started showing a loss of fuel economy right around 66,000 miles.  I also notice an intermittent “slightly” rough slow idle.  Engine seems a bit down on power also.  Drove 275 highway miles empty in good cool weather and could not hit 18mpg.  This truck is regularly in the upper teens to upper 20’s with the cruise set at 70 on that kind of trip.  For the record, I have not had an upper induction service performed yet.  But this seemed to set in pretty abruptly.  Doesn’t feel like running on 7 cylinders, and still has plenty power when you stand on it, but something’s just not right.

[tnchevy]

Bad gas at your last fill up maybe?

[swathdiver]

Low octane fuels lead to dirty injectors and fouled plugs.  Change the plugs and wires and drop in a couple of $.97 bottles of Super-Tech fuel injector cleaner from Walmart at your next fill.  Start running 93 or E85 if yours has a FlexFuel capability for 5 or 6 tanks.

[GM Gearhead]

Thanks guys!  I initially sided with a bad tank of fuel, but his has been ongoing for about 6 weeks and several tanks of fuel.  I generally run 87 octane fuel from a good reputable station that does a lot of business, so the fuel is always fresh.  I’m not disputing that it could be plugs, but I have yet to hear of anyone needing to replace them before 100,000 miles.  And wires too??  As for fuel treatment, I’m a former marine mechanic, so I just added a bottle of concentrated decarbonizing agent used in outboards and sterndrive marine engines.  The stuff does an awesome job!  Funny thing is, the weather perked up in the Midwest today and we hit a high of 61 degrees.  I actually started getting some mileage back as I put about 240 miles on the truck today.  The unknown thing is if it was the weather, or the treated fuel that I just put in it yesterday.  Time will tell!  The dealership offered to do an upper induction service for $179.00, so I may just schedule it and see what happens.

 

Thanks again!

[Aaron1085]

What did you put in the tank?


Sent from my iPhone using Tapatalk

[GM Gearhead]

 A product called Quikleen from Mercury Marine.  Mileage continued to increase today.  Filled up today and hard math showed the last tank 1.1mpg better than the previous.  Which begs the question, is the Quikleen working, or is the cold weather jacking with it somehow?

[Obscenejesster]

We had unseasonably cold weather in MD for a week or so. My mileage dropped and my engine didn't like it either.

Sent from my LG-H932 using Tapatalk

[swathdiver]

Seen a lot of posts about his lately across the internet.  Being in the south, it's not something we generally see.  But I did notice a drop in mileage going through a rain storm the other day.  Lost 1/2 mpg just hitting wet road and another 1 mpg in torrential rain.  That was something I never noticed before.

[gone_fishing]

Cold temps definitely lower economy. Up here in ND, it's been pretty cool lately. I'm lucky to get 17 MPG on the highway (70 MPH) when it's below zero. Over New Years I was traveling into a stiff win and temps were -10 to -15 below. Got about 14 MPG on the highway. This week it warmed back up to 35F and I was up to 19MPG. I'm typically around 20MPG in the summer.

[SNAFU88]

I experienced a similar sharp drop in MPG back in October. Went from a steady 17mpg in almost exclusively "around town" driving to 13-14mpg since.  Truck is a 2015 5.3 with 27k miles on it,  285/70-17 BFG KO2s on Pro Comp Wheels (85lbs per corner), and I use 93 Octane.  I was comfortably above 17mpg all summer and well into the 20mpg range on the highway.  Seen a few posts from fellow members about this so it seems the colder weather plus the "cold blend" gas used in northern states is likely to blame.

[Coopdeville]

Check the plug wires. They were falling off the coil packs on our 2015 collecting moisture and causing corrosion. They were manufactured without the retainer clips to hold them on. GM knows about this.

[tnchevy]

If your getting 1+mpg back with the warmer temps then there is your answer.  I usually get 2 to 3 mpg lower in the winter compared to the summer in my 2014 and the same is true for all my vehicles.