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Track Rat
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Discussion Starter #1
Hello everyone.

A few weeks ago I was halfway done with a 2000 mile road trip, when I got my oil changed and tires rotated at a dealer in St. Louis, MO (it was a dealer I purchased the car at). After, I noticed an immediate average drop of 10mpg, which is just sickening. By the time I verified this trend I was driving through Dallas, TX, so no way to go back to the dealer.

Things I've done to try and find a probable cause:

Checked the oil - it's full
Checked the air filter/intake tract - filter is pretty clean and no obstructions in intake tract
Did a used oil analysis - it's the right weight oil and everything looks good wear wise
Checked for loose or unplugged sensors/wires - everything is secured properly from what I can see
Checked tire pressure - it's set to recommended door pressures

The MIL is OFF, but I haven't attached a scan-tool to watch the live-data or check for pending/saved codes yet.

A bit about the car:

2014 Mazda 3 2.0L Skyactiv
About 23,000 miles
Oil changed every 5,000 w/castrol magnatec 0W-20 and either WIX or genuine Mazda filter
purchased in St. Louis, but it's located in Austin, TX now
Was getting an average of 37-39mpg before oil change, is now getting about 28mpg on average

Please help! Thank you!
 

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Working Class Hero
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Did you fill the tank with gas just before that service? I ask because a bad tank of gas can destroy your mileage.

Sent from the wrong side of the tracks.
 

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Track Rat
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Discussion Starter #3
I've filled it up over 7 times since this started, at different gas stations too. Average mileage has stayed crappy. :(
 

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Possibly the guys who did the tire rotation messed up and somehow caused your brakes to rub? Can't really think of anything else. Maybe have another dealer or shop look at it?

Also have your driving habits/setting changed? You said you were on a road trip, so if you suddenly switched to city driving 28 isn't unreasonable.

Also i don't know the geography of the place, but if the rest of the interstate is uphill or upwind you'll be surprised how much of a difference that can make. Or if you up your average speed on the freeway that can kill it too.
 

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Assuming that your engine is running the same as before your drop in fuel mileage could be caused by a bad rotation. Go for a drive 10+miles and feel for any hot areas, feel your tires, feel the center of each of your rims for heat from the brakes. If they all feel the same temperature move on.

Next you can check how your engine is running with an OBDII reader with a smartphone for about 30$. This will tell you your transmission temperature, Oil temperature and the community here could compare temperatures to a properly working 2014. With the same OBDII reader you can check if there are any issues with the oxygen sensors which also could cause bad gas mileage.

Far fetched issue could be the tech not properly installing the covers underneath the car. Check underneath for any hanging covers or loose covers that could get caught in the air while moving.

good luck
 

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Track Rat
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Discussion Starter #6
No dragging brakes, I'd hear, feel, or smell that. I've been driving for a while, competitively for about 5 years now too, so my driving habits are pretty well established, buts always a possibility. I was still on the hwy for at least a thousand miles to track mpg, so it wasn't a hwy/city thing. One thing I did think of is maybe eastern Texas and Arkansas has a lot of ethanol free gas. I could have been fulling up with ethanol free, then when we got to St. Louis and more populated places the gas had more ethanol in it.

Possibly the guys who did the tire rotation messed up and somehow caused your brakes to rub? Can't really think of anything else. Maybe have another dealer or shop look at it?

Also have your driving habits/setting changed? You said you were on a road trip, so if you suddenly switched to city driving 28 isn't unreasonable.

Also i don't know the geography of the place, but if the rest of the interstate is uphill or upwind you'll be surprised how much of a difference that can make. Or if you up your average speed on the freeway that can kill it too.
No loose under panels. I'm going to get an OBDII reader from Amazon and start data-logging what I can. I might just do an oil change for the hell of it. I always do my own service and her dad was just trying to be nice, but there's a reason I work on my own cars. So I know exactly what was done and what was used/put into them.

Assuming that your engine is running the same as before your drop in fuel mileage could be caused by a bad rotation. Go for a drive 10+miles and feel for any hot areas, feel your tires, feel the center of each of your rims for heat from the brakes. If they all feel the same temperature move on.

Next you can check how your engine is running with an OBDII reader with a smartphone for about 30$. This will tell you your transmission temperature, Oil temperature and the community here could compare temperatures to a properly working 2014. With the same OBDII reader you can check if there are any issues with the oxygen sensors which also could cause bad gas mileage.

Far fetched issue could be the tech not properly installing the covers underneath the car. Check underneath for any hanging covers or loose covers that could get caught in the air while moving.

good luck
Thank you for the ideas everyone. I'll update when I make progress.
 

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Wow, a 26% decrease in mileage is shocking given your attention to detail and experience. Other than what you've done / doing already, I believe I'd jack up each wheel and spin them to see if something is amiss, such as a wheel bearing or caliper drag.

Maybe the OBDII reader will show the problem. Maybe it will show a code, or fuel/air ratio problem, or excessive back pressure from a bad catalytic converter or ...??

Let us know what you find. Good luck!
 

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Hello everyone.

A few weeks ago I was halfway done with a 2000 mile road trip, when I got my oil changed and tires rotated at a dealer in St. Louis, MO (it was a dealer I purchased the car at). After, I noticed an immediate average drop of 10mpg, which is just sickening. By the time I verified this trend I was driving through Dallas, TX, so no way to go back to the dealer.
I just read this thread about a First service and he noticed they also updated his PCM. Did the dealership do any other service than oil & tires?
 

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From experience, ethanol free gas is only good for about 4-5% improvement (and totally unavailable in DFW and Austin). Sounds like a lot more at play. Maybe you picked up a 3s by accident? I get 24-25mpg.
 

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Are you manually measuring MPG? I mean, dividing how much you put at the gas station by the miles? without using the car computer MPG.
Apart from other advices, I would disconnect the battery and reconnect to reset the computer. Maybe for some reason something like a bad tank of gas messed up the system and it is not running at the right fuel/air ratio.
 

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Track Rat
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Discussion Starter #11
I would be furious if they flashed my PCM without telling me. Is there a timestamp stored in the computer with a log of updates, so I could see if they flashed it or not?
I just read this thread about a First service and he noticed they also updated his PCM. Did the dealership do any other service than oil & tires?
That's been mine as well, So while it could have contributed to a drastic change, I don't think it is the cause.
From experience, ethanol free gas is only good for about 4-5% improvement (and totally unavailable in DFW and Austin). Sounds like a lot more at play. Maybe you picked up a 3s by accident? I get 24-25mpg.
Yes, I'm calculating it with the trip meter and gallons filled.

I'll try resetting the PCM today. I'm also going to look into a clogged/broken cat.

The quest continues...
Are you manually measuring MPG? I mean, dividing how much you put at the gas station by the miles? without using the car computer MPG.
Apart from other advices, I would disconnect the battery and reconnect to reset the computer. Maybe for some reason something like a bad tank of gas messed up the system and it is not running at the right fuel/air ratio.
 

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Track Rat
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Discussion Starter #14
Tire pressure is at 34 psi, uoa shows the right right oil. I'm going to reset the computer today when I install my new aem filter. Thinking about just changing the oil so I know exactly what's in it. Haven't been able to track mpgs the past two tanks, cause my gf keeps forgetting to calculate and reset the trip meter, ugh.
 

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there are many ways that affect ur mileage....

1) temperature- drive in cold engine?
2) tires- a less-pressure tires = less mileage..
3) gas- gas is important since it is the fuel it burns...
4) elevation- thinner air, less mileage...
5) city vs country- did u go to city often recently??
6) dealer reset the computer - if u drove highway often before u take it to dealer, they reset it and the data is gone and it will take time for the computer to est the mileage back to normal
and many more....
 

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Does your GF brake with her left foot? It sounds like she is also driving the car and is a variable not discussed so far.


Sent from my iPhone using Tapatalk
 

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Could be that the tires where overinflated at first. Not so good for tires and handling but better mileage. They tend to ship the cars from Japan with to much pressure in the tires.
 

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I had my 5K miles service done a few weeks ago and also saw my average drop from a consistent 42.1 mpg to 40.2 mpg. I did not measure my tire pressure before the service, but could definitely notice a slight sag in the front tires (they are at 38 psi) so I tend to agree with comments about tire pressure. My tires were probably over inflated when shipped and when my tires were rotated and nitrogen filled at the 5K service they were inflated to the recommended max of of 38 psi. I am sure if I increase the pressure I would see a return to the mileage I was getting, but safety is essential so I am going to keep them at the recommended pressure.
 

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My tires were probably over inflated when shipped and when my tires were rotated and nitrogen filled at the 5K service they were inflated to the recommended max of of 38 psi. I am sure if I increase the pressure I would see a return to the mileage I was getting, but safety is essential so I am going to keep them at the recommended pressure.
I keep mine at 38 too, but the actual maximum pressure for any tire is printed on the tire's sidewall, not what the automaker recommends. The tire manufacturer is the one that knows what the maximum safe pressure for their product is. Automakers set their recommendations based on other factors, like comfort.
 

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I don't beleive, that tire pressure has significant factor to the consumption, if it is in the "normal" range. I drive my cars for 20 years now with the possible lowest tyre pressure (around 2 bar, 29 psi), to get the comfort. Checked many times with hihger pressures, but no measurable results. Of course, if you go over 2.8 bar / 40 psi, you will get some more mpg (1-3%), but at what price? Total discomfort (here in europe the road quality is various)
What helps: quality fuel, fresh/clean air filter, fresh engine oil, economy tyre. After the periodic service, I get 5% lower consumption in the first 5000 kms... My new Goodyear Ultragrip 9 snow tyres gives more mpg than the stock Toyo Nanoenergy summer ones!!! My average mpg is 45 (40% city, 40% highway, 20% main road)
 
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