Mileage dropped off suddenly

[Chad Miller]

I have a 2014 Silverado crew cab with a 5.3 L V8. About 4000 miles ago at around 68,000 miles, the mileage dropped off quickly from an average of 18.5 to a range around 14.5 to 15.5. I know it got cooler out with the fall and winter setting in however last winter I was still getting 17 miles per gallon. Is there something about these trucks when they hit  the 70k - 75k mile range that causes them to lose mileage more than previously?

[MN1(SW)]

Check all your filters? Air? Fuel? 

[300 Blackout]

A gradually degrading pre-cat O2 sensor will cause you to run rich. It’s function might not have degraded enough to trigger a check engine light yet.

[Black02Silverado]

Just happened or has it been like this through several tanks of fuel?   Just wondering if you got a bad batch of fuel. 

[1SLOW1500]

Yes there is. However the colder it is out side the denser the air so the pcm throws in little more fuel also the truck changes the fuel tables and runs a higher idle. 

[UnlimitedMatt]

I know where I'm at, they change the gasoline you purchase to the 'winter blend' that adversely affects your milage. 

[slj2008]

Buy good GAS , SeaFoam  or any other gas line treatment & KEEP your gas tank full . Truck's do run longer in winter ( warm-up time) . 

[Mike GMC]

Are you using remote start more and letting it warm up before you drive?  That is horrible for gas mileage.  I've noticed that mine gets horrible mileage until the engine warms up.  So the colder it is, the longer that takes and lower the mileage goes even though I don't let it sit and warm up.

[L86 All Terrain]

"Winter gas" blend is out now, it's just another way for the government and big oil to get more money from you every season. I have never had issues with gasoline in the winter. Many, many fuel filter issues with the diesels though, so winter diesel is a must. Cold, dense winter air allows the engine to make more power, which means you will burn more fuel along with the better air. Cold differential/transmission oil will create a parasitic loss as well, until you get the temps up your engine works harder to keep the truck moving. Generally people pull a few PSI out of the tires for traction in the winter which will increase drag. Many reasons.   

 

My 6.2L recently dropped 2mpg after a new transmission was installed, I also noticed it didn't want to kick down to V4 mode very often. This was in late October, I don't drive this truck in the winter so I will have to wait until spring to find out if it was a "learn" issue. 

[BigBadSierra]

That's what my 2014 and 2016 got.. the colder it was the worst it was.

[truckguy82]

2 hours ago, L86 All Terrain said:

"Winter gas" blend is out now, it's just another way for the government and big oil to get more money from you every season. I have never had issues with gasoline in the winter. Many, many fuel filter issues with the diesels though, so winter diesel is a must. Cold, dense winter air allows the engine to make more power, which means you will burn more fuel along with the better air. Cold differential/transmission oil will create a parasitic loss as well, until you get the temps up your engine works harder to keep the truck moving. Generally people pull a few PSI out of the tires for traction in the winter which will increase drag. Many reasons.   

 

My 6.2L recently dropped 2mpg after a new transmission was installed, I also noticed it didn't want to kick down to V4 mode very often. This was in late October, I don't drive this truck in the winter so I will have to wait until spring to find out if it was a "learn" issue. 

10 years ago while in VT my fuel line froze

 

So there is some truth to it

[Chad Miller]

Thanks for all the info guys. Today the truck bounced back in a 45 mile run I got 18.2 miles per gallon. One of the suggestions here is the pre cat O2 sensor may be degrading. I'm going to take it in and have them do their 36 point inspection on it because I run out of warranty in mid-January. I'm going to have them check that O2 sensor as well. Once I find the culprit I will report back on this post.

[Loco-diablo]

My 2017 mpg drops this time of year also... I attribute it to winter blend gas, using the remote start, and lower tire pressure due to colder ambient temps. 

['17 Sierra]

On 12/19/2018 at 11:57 AM, L86 All Terrain said:

"Winter gas" blend is out now, it's just another way for the government and big oil to get more money from you every season. I have never had issues with gasoline in the winter. Many, many fuel filter issues with the diesels though, so winter diesel is a must. Cold, dense winter air allows the engine to make more power, which means you will burn more fuel along with the better air. Cold differential/transmission oil will create a parasitic loss as well, until you get the temps up your engine works harder to keep the truck moving. Generally people pull a few PSI out of the tires for traction in the winter which will increase drag. Many reasons.   

 

My 6.2L recently dropped 2mpg after a new transmission was installed, I also noticed it didn't want to kick down to V4 mode very often. This was in late October, I don't drive this truck in the winter so I will have to wait until spring to find out if it was a "learn" issue. 

True that cold air is more dense than warm air and you CAN make more power but for a given volume of air there is a given amount of fuel (AFR). WOT you'll see an increase in power though and less MPG. Your ECM controls the AFR from data received from the O2 sensors and Mass Air Flow sensor to keep the specified fuel tables in check.

True on the parasitic drag too with your new trans.

Edited December 20, 2018 by '17 Sierra

[fbachman]

Yep, pretty standard behavior.

 

Cold weather + winter blend + possibly using remote start more frequently (I certainly do, rig parked in the open) = mileage degradation.

 

Depending on where you are, and our local gas station, the amount of Ethanol blended in will have a large effect as well. The more alcohol, the more your mileage degrades, as the heating value of ethanol is 80% that of gasoline, meaning, you get 80% the heat from the combustion of a fixed mass of ethanol than you will gasoline.

 

I don't remember what the laws are in particular, but it is my understanding that up to a certain percentage of ethanol, they don't even need to tell you. 

 

Case in point:

 

Years ago (in my 07 Sierra at the time), ran a few tanks of Sunoco, followed by a few tanks of BP (all 91 Octane). The mileage on Sunoco was verifiable less than that of the BP, most other things remaining relatively constant. Did a bit of research, and learned that the Sunoco (around here) had quite a bit more ethanol in it, at the time, than the BP did.

 

So, that's another possibility.

 

I have noticed in the winter, that although mileage is generally lower for all of the legitimate reasons posted above, that I did see mileage differences between various gasoline brands. Experiment a little. You won't get "summer mileage" to be sure, but you may see a bit of an improvement.

 

 

Edited December 21, 2018 by fbachman