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Sudden loss of fuel economy - 2014 Chevy Silverado 5.3 LT - dropped 7m


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Recently (started three months ago) - I have noticed a sudden loss of fuel economy in my truck. I drive 500 miles a week ALL highway. I have gone through 2 winters on it (bought with 20k miles and now has 70k miles).

 

In september 2016 - I noticed my highway milage dropping. I used to get 21-22mpg on my 170 mile commute (all highway 70mph). It performed like this since I bought. I started seeing 17-18mpg highway, and now I get no better than 15mpg highway.

 

My average over 50k miles driving was 20mpg

My average over last 3 months and 4000 miles is 14.5mpg

 

While on the freeway at 70mpg - it gets 14-15mpg.

 

This is very frustrating because for past two months I have been troubleshooting with my dealership. I have extended bumper to bumper and full drivetrain warranty - none of which covers this issue!!! They can't tell me whats wrong with it and seem unable to diagnose the cause.

 

We have tried the following:

-Put different stock tires on the vehicle and drove for two weeks, no difference

-Put a MAF sensor from a different (new) silverado and drove for two weeks - no difference

-Tested for fuel consuption - no excessive use of oil (very little to none being burnt)

-Oil changes - no difference

-New stock airfilter and clean MAF sensor - no difference

-I use only top tier gas and oil

-No tire wear visible

-Fuel trims appear to be good

-No diagnostic codes showing up

 

I know there are many variables for fuel economy, but we have look at everything we could and have ruled out most everything.

 

The dealership is of no help and says GM has no documention on this issue elsewhere.

 

I have talked with many auto professionals and they all say this sounds bizarre or offer up that I have a crap engine.

 

Regardless I have full bumper to bumper/drive train warranty and its of no use!!!!

 

Does anyone offer up what I should take for next steps? I mean, I used to own a 1971 Chevy C10 3 on the tree, that got better mpg than my 2014 Silverado. Its rather unbelievable

 

Where do I go? Who can properly diagnose the issue?

 

Any help appreciated! Thank you!

 

-Darren

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Have they inspected your catalytic converter for possible collapse or meltdown? Did they check engine load at cruising speed. If you don't have sticky brakes bogging you down and you didn't change differential fluids for a higher number the only thing left is plugged exhaust. Get this resolved soon or you'll be burning exhaust valves. Mine got harder on fuel when the colder temps came around and winter gas isn't has efficient either. But I'm still getting better than your numbers. Coming back today I averaged 19.6 MPGUS ( 12 l/100Kms )@ 32°F ( 0°C ).

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Ihave put fuel injection cleaner through twice to no affect. Does not do any good for direct injection.

 

I brought it in to a third party today to do an induction cleaning through the air intake. well see if it does any good.

 

Can you elaborate on the possible exhaust issues? I am trying to get my dealership to think outside the box.

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What brand and octane of fuel are you running? Also what brand and viscosity of oil are you running? I know you said top tier, but let's be specific here. What brand of additives have you used, both oil and gas? What is your MPG @ 65MPH?

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Top tier...I dont have loyalties and fill up all over. In my area top tier is sold by holiday and super america gas stations. Have tried two different oils over past months. Full synthetic and also a blended ow-20. currently have dexos ow-20 the dealership put in.

 

@65 mph on my gauges it states I am getting qbout 15mpg. I can hand calculate and it is accurate. I used to get over 500 miles per tank and now getting about 390 per tank, approximatley.

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My first thought was winter c/w winter gas as I've also noticed a quite a hit in mpg lately. My numbers aren't nearly as dramatic as yours. I believe Coby7 's comment about brakes should be followed up. The brakes would be second on my list of things to check, next to fuel. Keep us posted!

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My dealer and the third party who just did my BG Induction cleaning, have not mentioned the cats yet. and we have had hour long conversations trying to think about anything and everything.

 

My guess is the dealer is saying, there are no codes or sensors telling us this, so it must not be it.

 

Should I bring it to an exhaust shop and have them take a look at the cat?

 

appreciate all the input. I have been all over this forum this morning and these threads turninto everyone offering what they get for mpg. I dont want that to happen here, its not relevant. Thank you for keeping the troubleshooting going!!!

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Do you ever use your emergency brake? Mine got stuck in my '03 Silverado a couple of years back and I experienced the same issue. If the emergency brake is stuck or your one of your calipers is dragging on the rotors it will definitely drop your mpg. When you drive get up to speed and put it into neutral and see if the truck coasts or slows down much faster. or find a steep driveway or hill on a backstreet and come to a standstill and put it in N, see if the truck rolls freely or feels like there is a drag. good luck

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I'm still hung on your cats, they will not give a code until they absolutely plug up then you will have a discrepancy between the upstream and the downstream HO²s but if they have started to meltdown they will obstruct but will remain within the specified window. This will cause your engine load to increase and if you have AFM your engine will never go into cylinder management therefore increasing fuel consumption. These can easily be checked by pulling the upstream HO²s and inserting a cylinder inspection camera like Snap-On. If the honeycombs are all droopy or have collapsed and melted look no further.

 

Here are a few snap shot of my 5.3l, one at idle and two at 65 mph (107 Kms/h ), in the lower bottom half you can see the load in %

 

Idle

2016-12-24%2017.59.51_zpskdsgghz4.png

 

In V4

 

2016-12-24%2017.53.01_zpsdshcv1iu.png

 

And in V8

 

2016-12-24%2017.52.48_zpsky3hygpb.png

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Coby7, what monitor/tuner setup do you have. I might look into that.

 

UPDATE:

Issue seems to have been mitigated, good news. Yesterday I had the BG Induction air intake cleaning through the throttle body. 150 dollar service. Didnt drive it last night.

 

Today after some suggestions. I looked at the ebrake and it appeared to have been fully disengaged.Regardless I engaged and disengaged the Ebrake fully. I never use it and it.

 

I then just drove 35 miles and reported 20.5mpg on the last 25mile screen. Back to what I would consider normal!!

 

It was either the ebrake or the induction cleaning. I can say that the ebrake has not been engaged by me since owing thr vehicle. I never felt any brake drag either.

 

I think it was this induction cleaning.

Thoughts?

 

Ill definitly have the cats looked at anyways.

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