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Discussion Starter · #1 · (Edited)
I know I mentioned this awhile back when I hit 30K, but I've blamed it off on one thing or another for quite some time. This weekend the nail was driven into the coffin as I took a trip I've done many times before and clocked a fantastic... 33MPG for the trip. In the past that has been anywhere from 37-40 at the same speeds sustained. Is this possibly buildup on the valves over time? (TPMS has been being stupid, but tire pressures are 36PSI cold in all 4, checking every morning).
On my daily drive I had been returning (and griping about mind you) 32-33 (down from 35-36)... now more recently down to 29-30. Again, I've been finding excuses to write it off... but is it time to bring this up to the dealership service and see about having the valves checked for build-up and a general run down of other things?

Rest assured, I want to blame my own driving habits as well, but the other cars haven't taken a sudden slump in MPG they're both as good as ever clocking in 31-32 in the miata and 24-26 in the formula on trips. (a good portion of the trip I reference above in fact..)


Update 1 (Post techron additive):
I'm not far into the first fill after the additive, but so far performance hasn't dropped from the tank WITH the additive, and MPG is up back around 34MPG average so far this tank. That's a near 5MPG (re)gain indicated so far.. but I'm only 100 miles in, so we shall see :)
If this holds up, it's safe to say the valve timing definitely allows for valve cleaning, and a little boost every now and then with something like the techron/seafoam/etc wouldn't be a bad idea. For this to be effective, fuel must get to the top side of the valve, thus the concern of it not working at all on a DI engine.

Update 2 - 4 tanks in
Back down to averaging around 32MPG... still driving 80/20 hwy/city...

update 3 - Long trip fuel mileage still down, but up from what it had been.
To expand on that a bit, I recently took a trip I've done a couple other times when the car was new to the tune of 39-41MPG averages. This last time I got 36, and ended at 33 for the tank... vs typically ending with 39 on the tank. I think "blah" sums it up pretty well. At least gas is cheap again :/
 

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I took a drive from Columbus OH to Indianapolis this weekend, I started with avg @ 30.5 (over 1/4 tank used so far) and got there with 29.5 MPG AVG! i-70 the whole way cruise control on, just the wife and I in the car. On the way back we avg 33 mpg on a fresh tank. 15k on the odo and have taken this trip before and always averaged close to 37-40. Not sure what is going on but it has taken a noticeable dip. I love driving the car and I do not drive it nicely on the daily, but on a trip i like seeing the big numbers on the MPG menu.
 

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Get the service manual, pull the intake manifold and look at the intake valves for yourself...and take photos if there is carbon buildup. Mazda says that valve overlap prevents this, but I'm not sold given other people saying they have benefited from sea foam cleaner.

Also do direct injectors ever need to be cleaned? I know somebody else here said their car benefitted from some injection cleaner.

Also are you running a slippery oil like the factory oil? Not all 0-20s are created equal.

Freakish of all, you could check the engine compression to make sure there isn't an issue with combustion chamber sealing.
 

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This time of year my mileage really takes a dive... I've attributed it to the following things:

1. Increased air resistance with colder temperatures. The air (being ever so slightly more dense) is ever so slightly more difficult to travel through. Although more dense air enters the combustion chambers, the car still has to travel through it, requiring matching fuel to burn that dense air and even more fuel to propel the body through it
2. Colder tires - increased rolling resistance compared to summer
3. The engine requiring more thermal energy to run the same operating temps as the summer (up to 60F cooler)
4. Any aerodynamic modifications... the muffler strikes me as a key component for under-tray airflow. If you've made this mod, the colder temps will exasperate your fuel economy as exiting air will cause a pickup-truck-like venturi under the rear bumper
5. Increased warm up times, requiring different fuel maps for longer periods
6. Potholes jarring your alignment (particularly toe) to out of spec. Out of spec toe readings will mess it up.

I don't think it's out of line for a 15% increase in fuel consumption given all of these factors (whichever apply to you)... 15% off of 37mpg, leaving you around 31.5mpg.

Paying for the crushed walnut treatment for the intake valves is one option, (you would have noticeable power loss from now with gummed up intakes... have you lost a lot of power??) however I'd probably look to nature/aerodynamics/drag to be the cause...
 

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Fuel blends are seasonal and can have a quite-material impact on mileage. In addition warm-up times as noted are longer and until the ECU goes into closed-loop operation (when the catalyst lights off) mileage will SUCK. Short trip driving in cold weather can murder your average fuel economy between these factors.
 

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Ya, I've gone from an average 36-38 to 33 since the stations switched to winter blend fuel and I threw on snow tires. Guessing that warming up the car for 5 min at work at night isn't helping much either! LOL!
 

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Discussion Starter · #7 ·
you guys seem to mis-understand what I've asked. This has not happened just since the switch to winter blends. This has been an on-going continuously detrimental problem. My last trip in the same conditions was also in winter with ambient temps in the 40s-50s with winter fuels... (roughly same time of the year). My gas mileage has been on a down-trend since 30K about.. (4 months ago?). I'm not grumbling about a 3-4MPG drop, this is nearly a 10MPG drop, in the same conditions on the same trip. As I said, I understand changes do occur, and I've been writing off the down-trending MPG for quite some time...

As for a couple of the other factors, car is still 100% stock and alignment noted as within spec at least OCI/inspection - 32K miles... oh yes.. fresh oil change too 2K miles on it, that's never been enough to adversely affect MPG before on it's own, usually detriment begins around 6-7K.

Dis-regard warm-up times to some affect.. this was 4 hours of driving non-stop. (on the particular shoved me over the edge crap MPG returned at least).
Again, overall I'm down 5MPG ON AVERAGE vs last year per my records.
 

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Discussion Starter · #8 ·
Get the service manual, pull the intake manifold and look at the intake valves for yourself...and take photos if there is carbon buildup. Mazda says that valve overlap prevents this, but I'm not sold given other people saying they have benefited from sea foam cleaner.

Also do direct injectors ever need to be cleaned? I know somebody else here said their car benefitted from some injection cleaner.

Also are you running a slippery oil like the factory oil? Not all 0-20s are created equal.

Freakish of all, you could check the engine compression to make sure there isn't an issue with combustion chamber sealing.

There's some points I hadn't really thought of.. I do normally run chevron cleaner every few tanks.. I've avoided that with this car, as I thought it was a complete waste. Maybe I shouldn't have? I also thought the same on sea-foam with a DI car, perhaps that's needed as well, but most comparisons using it even as often as every 5K, by 30K there is still crud on the valves.


Compression... If anything the car has more pep now more than EVER. If it's down on compression and performing better.. so be it, LOL.
 

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10mpg is a 20-25% decline.

Something's wrong with the vehicle; I'd check for a drag sources (e.g. a dragging brake caliper) for starters.

My last 4 tanks have been down a few mpg but I've had a bike carrier on the back with bikes on it -- and that hits me for ~3-4mpg at 75mph. Drag matters, especially at highway speeds!
 

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Discussion Starter · #10 ·
10mpg is a 20-25% decline.

Something's wrong with the vehicle; I'd check for a drag sources (e.g. a dragging brake caliper) for starters.

My last 4 tanks have been down a few mpg but I've had a bike carrier on the back with bikes on it -- and that hits me for ~3-4mpg at 75mph. Drag matters, especially at highway speeds!
Hadn't thought of that ticker, thanks. I haven't noticed any dragging sounds, or anything overtly obvious hanging down (where I park it's usually obvious). However, I WILL take a quick look under/around the car for anything that sticks out that could be causing a problem! (would be quite fun to call the dealer griping about MPG to find a splash guard somewhere has come loose -.-)
 

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@RacingTiger03

Are you using 87 octane fuel? I notice a big drop with 93. I use Chevron fuel system cleaner every couple k to keep things clean...it does seem to help maintain restore mpgs with this engine.

Id check to make sure your brakes are not dragging.
 

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Pick the rear wheels up one at a time (jack time!); if you have a dragging caliper it will be instantly obvious when you attempt to rotate the tire manually. Fronts are trickier because normal driveline drag makes it difficult to discern the difference. This is part of my regular inspection when I rotate my tires with oil changes.

I've got nearly 20k miles on my 2015 Sport manual and my "best ever" tank was a few kilomiles back (just over 41mpg actual, measured by miles covered divided by fill volume.) I run 87 almost all the time since I tested 93; premium fuel returned a bit less mileage although IMHO the engine ran a bit smoother with more throttle responsiveness in the light throttle and lower-midrange area. IMHO not worth the materially higher cost for daily driving.

With the bike rack on back cruising @ 75-80mph I lose quite a bit over the same load but "clean" aerodynamically (e.g. bike in the TRUNK instead of on a rack) -- we're talking going from ~35 -> 30ish over the same road and comparable conditions. That's a huge change and yet the bike and rack are both in the slipsteam (hitch-mounted platform rack; nothing extends beyond the profile of the vehicle) so don't ever believe that minor drag changes will not murder your mileage, especially on the highway, because it does.
 

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Discussion Starter · #13 · (Edited)
Also yes, Using just 87 octane when the weather got colder. After 50* ambient it's not worth the added fuel cost IMO.. but 70+ it's an easy choice to use premium. Brakes feel to be working well, and there is no excess wear on any one rotor... I will double check the free-play though to be certain..

I also got my best MPG's around 20-25K miles ticker.. held steady to about 30K and it's been downhill ever so slightly ever since. Much like you I hit 41+ MPG on a trip to myrtle beach. Wound up with 39.6 calculated out for the trip after a day of city driving to finish off the tank (50 miles out of the 620) as you can imagine a lot of that city driving was sitting, lol.
 

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This vehicle seems extremely sensitive to ambient temperature in regards to achieving good mpg's. I can never approach my summer full tank averages of mid to high 30s mpg after the temps drop below forty degrees.

I'll be lucky to get a tank average over 33mpg during the winter months with my 2014 Mazda6, and I never allow my cars to warm up. Start and go immediately, regardless of the temps.
 

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Over the past few weeks, I've noticed a considerable drop in mpgs, from about 37mpg down to 32mpg. I don't remember having this drop in mpgs in my 2014, but then again I didn't have to drive that much. My commute has changed from a 20 mile round trip to a 60 mile round trip due to a move a 5 weeks ago.
 

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did a 35mile run to the dealer yesterday and hit 39 mpg without much effort, average 65-70mph, 14k miles
 

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Hit a lowly 29MPG on a 200 or so mile trip this past Friday. Probably due to wind but man did that kill over half a tank of gas. I'm at about 17k miles.
 

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Discussion Starter · #18 ·
Over the past few weeks, I've noticed a considerable drop in mpgs, from about 37mpg down to 32mpg. I don't remember having this drop in mpgs in my 2014, but then again I didn't have to drive that much. My commute has changed from a 20 mile round trip to a 60 mile round trip due to a move a 5 weeks ago.
that sounds more in line with what I've been experiencing, lol. It's not a happy thing... but at least it's not JUST me.. :/ With yours being newer too, that makes me feel a little better about it, I think....
 

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Nothing is wrong, all vehicles get less mpg in the winter. Even the Prius drops down to almost 38 to 40mpg in the winter. From the typical 45+ mpg it gets all year around.

Every since I llowered my car and got the, wheels I have not seen anything over 29.8 mpg...lol
 

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Discussion Starter · #20 ·
you do realize I'm talking about getting 5-6MPG less than I got in the same/similiar conditions last year vs this year? I think a lot of people are missing that point...
My normal winter drop for this car was 2 MPG.
How many times has that been said now?!
 
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