HUGE MPG drop after routine tune up?

GMJunkie7 · January 22, 2025

Hey all,

Here's the story. I've got a 2016 1500 silverado highcountry 5.3 that has for the most part been good to me ( except for the hard shifts and transmission issues alot encounter)

Recently, I gave her the 100k mile tune up. As part of this, I did brakes, and spark plugs. My local Advanced didnt have AC Delco, so I used the "correct" replacement NGK plugs.

As part of the brake install ( since I tow I installed powerstop brakes) I had to do a brake in procedure involving accelerating to 40-50 mph and doing some hard braking then light braking, then 10 minute ride to cool them down. During this brake in procedure, I got a check engine light. Turned out, I got a bummed spark plug on my cylinder 7. No biggie, replaced the plug and went on my merry way.

However, since I replaced the brakes and spark plugs, I have gotten HORRIBLE MPG. I used to average about 22 on the highway, 16-17 city, and 18 ish overall. Now I am lucky if I'm getting 15 on the highway after shes warmed up and I've been driving for 15-20 minutes at a steady 65-70 with cruise control on. I am ALWAYS babying the gas trying to maximize fuel economy, but no matter how easy I drive, I still get horrible gas milage. I do live in Nebraska, and it has been cold here... but this issue started while it was still 50-60 degrees out, and I have never gotten this bad of gas milage in the winter. Sure it might drop down to 19 on the highway from 22, but never down to 14.

One additional thing to note is about 1-2 weeks after the tune up, I did recently get a check engine for the trans temp sensor. I cleared it, and it has yet to come back, but I have noticed the trans temp on the dash is hopping around quite a bit. Could this trans temp sensor have anything to do with the poor MPG? Maybe the trans isn't behaving as it should due to this faulty sensor? Think the spark plugs could be the problem? I used to get north of 450 mile per tank, and recently have barely been able to get 320.

Any Ideas would be appreciated!

CamGTP · January 22, 2025

If I was trying to figure it out I'd be looking at a data log to see how the engine is running on the freeway. Like watching various sensors and fuel trims to see what they are reading.

If you haven't ever cleaned the MAF sensor, I would do that too.

PhilB · January 22, 2025

Ditto on what CamGTP said. Plus since you did a brake job, I would also check to make sure you don't have a brake dragging.

If you still have the old plugs, you could put them back in to see if your MPGs go back up. If the MPGs do go back up, then the NGK plugs are the issue.

Just a thought...

asilverblazer · January 22, 2025

In addition to the above:

The transmission temperature sensor, if it is bad, could be preventing torque converter clutch lock up; this would lower mpgs, especially on the highway.

GMJunkie7 · January 23, 2025

4 hours ago, PhilB said:

Ditto on what CamGTP said. Plus since you did a brake job, I would also check to make sure you don't have a brake dragging.

If you still have the old plugs, you could put them back in to see if your MPGs go back up. If the MPGs do go back up, then the NGK plugs are the issue.

Just a thought...

Unfortunately I dont have the old plugs, otherwise I would do just that. I tossed them with all the packaging from the other components.

I did think about the brake dragging, but I have checked the rotor temps after a long ride and don't seem to have any big differences in temperature. I'm also not putting any more foot into the pedal, it just seems like as soon as i rest my foot on the pedal the instant MPG meter drops to below 10, even during slow acceleration.

I will for sure try the MAF sensor and report back. Not sure how to look into the data log but I am sure I can figure it out.

4 hours ago, asilverblazer said:

In addition to the above:

The transmission temperature sensor, if it is bad, could be preventing torque converter clutch lock up; this would lower mpgs, especially on the highway.

This may make the most sense to me. its funny, sometimes it seems like all of a sudden I am getting better gas mileage again ( judging by the instant meter) and I jump back to the trans temp reading and it shows a steady temp. Almost like sometimes the sensor does behave correctly, and when it does, that's when the mpgs go back up, but it could just be my imagination... that instant mpg meter doesnt tell the whole story.

reardiff · January 23, 2025

If you have a 2016 with 6.2, there's extended coverage on the trans temp sensor. GM released a special coverage that extends the warranty to repair this, up to 10 years or 120,000 miles. It's special coverage #N192291660. This isn't a cheap fix, mine went out not long ago and was $900 out of pocket. They have to drop the pan and replace the sensor and harness. For whatever reason the special coverage only applies to model year 2016. Although I'm not sure this has anything to do with your MPG issue, I had the same as you with erratic trans temp readings then eventually a check engine light that would come on and off but didn't seem to change MPG.

Here's a separate post on it you may find helpful.

newdude · January 23, 2025

Are you getting any codes for transmission temp sensor fault?

If you have no brake drag, and the transmission temp sensor isn't at fault, I'd get a set of OEM plugs. Use the old # 12622441 and not the new one 19459511. Don't get an "OE equivalent".

GMJunkie7 · January 23, 2025

2 hours ago, newdude said:

Are you getting any codes for transmission temp sensor fault?

If you have no brake drag, and the transmission temp sensor isn't at fault, I'd get a set of OEM plugs. Use the old # 12622441 and not the new one 19459511. Don't get an "OE equivalent".

I did have a code for the trans temp P0711/ P0700 When the codes were there, the shifting was very erratic. I cleared the codes to see if they would return, and they have not. It's been almost 3 weeks since those codes popped up and then were cleared. Shifting has returned to normal, but the trans temp display sometimes bounces +- 5 to 7 degrees every few seconds.

I have a heard time believing the NGK plugs could be at fault, but again, there's not much else it could be I suppose.

newdude · January 23, 2025

48 minutes ago, GMJunkie7 said:

I did have a code for the trans temp P0711/ P0700 When the codes were there, the shifting was very erratic. I cleared the codes to see if they would return, and they have not. It's been almost 3 weeks since those codes popped up and then were cleared. Shifting has returned to normal, but the trans temp display sometimes bounces +- 5 to 7 degrees every few seconds.

I have a heard time believing the NGK plugs could be at fault, but again, there's not much else it could be I suppose.

Start with the P0711 first then. As someone else mentioned already the converter might not be locking due to it not knowing the fluid temp which could decrease MPG.

As for the plugs, you'd be surprised. Modern GMs are VERY sensitive to aftermarket engine components. Plugs, wires, MAF, MAP and especially O2 sensors.