15l is not good, but it's also very possible. If you sit static in traffic, then you are burning fuel and traveling 0km. Totally depends how long you sit for and how heavy the traffic and how often you accelerate and brake.

 

Mine has i-stop (which hasn't worked for at least a year) and on highway with cruise control set to 90km/h I get 20.2km/l (translates to 47mpg). I do have the 2.0L 2012 Mazda 3. However when my foot feels heavy or when using my paddles to get through traffic fast, this gets halved.

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that's definitely a little high. despite what people tell you, the filters don't really have much of any effect on fuel economy. they might if they were REALLY trashed but that would take a lot. filters used to have an effect with carburetors but as soon as we started using fuel injection and sensors got better, all that happens is you lose some power in the higher RPM range.

worth knowing if you have manual or auto trans. tell us if there is anything unusual going on. noises? strange behavior? have you checked for ECU codes? any mods we should know about?

you're generally looking for anything that would waste engine power. a slipping belt, dragging brakes (the rear calipers are known to seize), fuel leak, misfires just to name a few. whats your mileage? was regular maintenance kept up? badly worn spark plugs could be a problem.

although very unlikely, the intake valves could be getting clogged up with carbon. i would leave that to last on the list to check. it's not really a problem on these cars but is not impossible. i'd guess usually with only driving short trips in the city would this ever be a problem and even then that would be rare.

give us all the information you can in as much detail as you can. fuel economy problems are difficult to sort out.

 

that's definitely a little high. despite what people tell you, the filters don't really have much of any effect on fuel economy. they might if they were REALLY trashed but that would take a lot. filters used to have an effect with carburetors but as soon as we started using fuel injection and sensors got better, all that happens is you lose some power in the higher RPM range.

worth knowing if you have manual or auto trans. tell us if there is anything unusual going on. noises? strange behavior? have you checked for ECU codes? any mods we should know about?

you're generally looking for anything that would waste engine power. a slipping belt, dragging brakes (the rear calipers are known to seize), fuel leak, misfires just to name a few. whats your mileage? was regular maintenance kept up? badly worn spark plugs could be a problem.

although very unlikely, the intake valves could be getting clogged up with carbon. i would leave that to last on the list to check. it's not really a problem on these cars but is not impossible. i'd guess usually with only driving short trips in the city would this ever be a problem and even then that would be rare.

give us all the information you can in as much detail as you can. fuel economy problems are difficult to sort out.

Actually a few days ago I noticed that the car is pulling to the left, it might be a seized break caliper but it also can be just a bad alignment, I will definitely check both of those as soon as possible.

For more details I can add that my Mazda came with automatic transmission. Before I bought this car I check it in local Mazda service, there were no ECU codes. Mechanics listened to the engine, they didn't hear anything strange neither, everything seemed fine. The car has 50 000 miles on it and as I said, I bought it a week ago so I didn't really maintained it yet, but last owner did every maintenance it needed, everything was checked and changed regularly.
P.S. It doesn't have any mods installed, everything is stock. It's just a car for everyday driving, which gets 15mpg in the city)).

 

You have only had the car a week...how are you determining your fuel consumption? Don't use the numbers on the dash display. Fill the tank and do the math.

 

Of course I'm not looking at the dash numbers, 5 days ago I filled the gas tank all the way up, drove the car for 100km(=62mi) and again filled it up until it was completely full. I did this to see how much fuel it consumes per 100km and the number was 16 litres.(=4.2gallons). Right now I'm trying another method. last time when I fully filled the car I also reseted a trip odometer. as you know Mazda 3 has 9 "Big Squares" and 6 "Small rectangles" on dashboard for fuel indication. I already drove 220km and Squares went down from 9 to 2. Mazda 3's tank size is 50L, each of "Big Squares" are equal to 5L. 5 x 7 = 35 | 35 / 220 = 0.159 | 0.159 x 100 = 15.9. and this is the math I'm frustrated with.

 

You can't really figure fuel consumption by driving a few miles or counting the lines on the fuel gauge. Neither way is real accurate.
The way the rest of the world has figured fuel consumption for years goes like this -
Fill the tank, record the odometer reading. Drive the car until the tank is getting towards empty again, fill it and record the odometer reading. Figure out how many miles the car drove between fill-ups. Divide the distance traveled by the amount of fuel used.
If you still only get 15 mpg, there is a problem. A bad coolant temp sensor could do this, or an O2 sensor that isn't working right, or a malfunctioning MAF sensor, or a number of other things or a combination of things, including poor quality gasoline.
Its also possible that sitting in traffic will lower your mileage considerably, more than you may realize. The reality is 30 mpg is a bit of an overstatement for your car unless you do some highway driving too. Constant stop and go is a killer for Skyactiv gas mileage.

 

I've always felt that because there are so many variables in "city" driving, comparing fuel mileage is not very informative. These variables include number of start-stop red lights and traffic blockages per mile, average speed, average distance per cold start, minutes of engine idling per trip, acceleration technique, braking technique, etc.

In contrast, I think that comparing highway fuel mileage is much more informative, with the main variables being average vehicle speed and, related, average wind speed and type (direction of wind relative to vehicle direction.) When comparing highway (especially Interstate) mileage, it is relatively easy to note things such as average speed and what the wind was doing, which is important to consider since they can be quite significant to the results.

In my case, my average fuel mileage to date is almost 45mpg for my '15 Mazda3 automatic. The reason it is so high: I have used it almost exclusively as a commuter car driven 90%+ on 200-mile nonstop highway trips with maybe 10 red lights per trip. I don't exceed the listed speed limit, my average speed is probably around 55 mph, and I'm an easy driver overall and don't compete in the highway "wolf packs" that force a driver to constantly accommodate the crowd.

Although the OP's 15mpg is probably at the very bottom of what one would expect from a Mazda3 in bad city conditions, it is not an implausible value. However, if it were me, I'd investigate for any vehicle problems as noted above, such as brake malfunction. If it's not such a mechanical problem, then it has to be city driving conditions and techniques.

 

I need some pointers as well. 2014 Mazda 3 2.0.

180km on the clock.

MAF + throttle body cleaned, new air filter and spark plugs listed on the manual.

The only thing not done is the cleaning of the intake valves, could that be it?


Mines a lot worse.

 

I need some pointers as well. 2014 Mazda 3 2.0.

180km on the clock.

MAF + throttle body cleaned, new air filter and spark plugs listed on the manual.

The only thing not done is the cleaning of the intake valves, could that be it?


Mines a lot worse.
View attachment 298545

so converting that you're getting around 11L/100km. i get about 7, but that's almost entirely highway. it does get better if i slow down, even under 6.

seems so far you're throwing everything at the wall to see what sticks. this is a terrible way to solve your problem.

with intake valves, its possible but highly unlikely. although our motors are direct injecton they're not like the early ones at all. we have no way of knowing if it is the intake valves without more information and eliminating other possibilities. read my old thread here for more info: https://www.mazda3revolution.com/threads/intake-valve-carbon-investigation.235229/

first thing is what is your highway/city driving split like? if you're predominately city, your fuel economy is not going to be great and that is normal.

you've done the plugs, ensure they're the right ones. our motors require iridium plugs and they're not cheap at about $20/plug give or take. where were they purchased? we have seen counterfeit plugs here before.

air filter doesn't affect fuel economy, only power. cleaning MAF and TB likely not required nor will it help. if it does it may be a sign that something else is going on. sticking brakes....let me guess, mexico built and it was the rear brakes? we can talk more about that one...

getting ECU data would be helpful as well as any other possible symptoms. CEL or not, a code scanner is required to check for logged codes at the very least. things like O2 sensor data would be helpful.

how about an exhaust leak? the flex pipe can break (for example) and that will have an effect. thermostat stuck open would take far longer to warm up the motor, or may not warm up fully at all. so many options.

more info is better.

 

LTFT seems fine....intake problem would include loss of power. significantly so. our cars build up hard carbon and not a lot of it. it needs to be picked out, the blasting will only do so much. we did have a member who soaked the valves in something like carb cleaner. the intake cleaning chemicals you spray in while the motor runs will not help carbon buildup.

that being said, you're wasting your money with the intake carbon but hey it's yours to waste.

by "bulletin rear brake job" i can only assume you mean the recall from...years ago. a service bulletin and recall are very different things. the recall was the boots on the rear parking brake cable. they didn't seal right and could cause the caliper to seize. if corroded, you're going to get to calipers. HOWEVER.....this is not the issue i speak of in my previous reply. read this one: https://www.mazda3revolution.com/threads/how-to-solve-rear-brakes-seizing.239565/unread

the result of that was instead of my rear brakes lasting only about 45k kms they now last over 120k kms. that is not a typo.

what was the previous mileage you had with this? if you're mostly in the city, your numbers aren't bad at all.

just a fun one, here's how to fix the nasty stains down the side of the front doors from the mirrors. it's a mexico thing. https://www.mazda3revolution.com/threads/how-to-fix-white-stains-from-the-side-mirrors.241270/unread

 

LTFT seems fine....intake problem would include loss of power. significantly so. our cars build up hard carbon and not a lot of it. it needs to be picked out, the blasting will only do so much. we did have a member who soaked the valves in something like carb cleaner. the intake cleaning chemicals you spray in while the motor runs will not help carbon buildup.

that being said, you're wasting your money with the intake carbon but hey it's yours to waste.

by "bulletin rear brake job" i can only assume you mean the recall from...years ago. a service bulletin and recall are very different things. the recall was the boots on the rear parking brake cable. they didn't seal right and could cause the caliper to seize. if corroded, you're going to get to calipers. HOWEVER.....this is not the issue i speak of in my previous reply. read this one: https://www.mazda3revolution.com/threads/how-to-solve-rear-brakes-seizing.239565/unread

the result of that was instead of my rear brakes lasting only about 45k kms they now last over 120k kms. that is not a typo.

what was the previous mileage you had with this? if you're mostly in the city, your numbers aren't bad at all.

just a fun one, here's how to fix the nasty stains down the side of the front doors from the mirrors. it's a mexico thing. https://www.mazda3revolution.com/threads/how-to-fix-white-stains-from-the-side-mirrors.241270/unread

Sparkplugs were the ones mentioned in the manual, I got it from Napa, a local parts store.

The rear brakes were serviced, a month ago. Rust build up was exactly the same thing I dealt with, in my case I used a dremal to clean out the cavity, and purposely ate a bit of the metal as well for the same reason, however I ended up replacing that caliper. I will be servicing the brakes on this car every summer, provided I live in salt city. (For the upcoming summer, I am looking to do all the brakes rotors + pads from Mazda to being everything back to factory)

Regarding the mirror, I am going to compounding and polishing the entire car once I am done touching up a few more rock chips and scratches, and claying the 10-year-old paint first. I am not bothering with the mirror stains as (Compounding will remove them), one of them is already after market, the other is original, however I don't intend on keeping either mirror in the long run, they'll get swapped out for heated ones. I am just waiting on my local salvage yard to get more Mazda 3s, the current ones are already stripped out/vandalized. The lack of heated mirror was annoying from day one during rain, and it'll likely get worse during the first snowfall soon. I'll be picking up a steering wheel with cruise control as well, swapping out the bare-bones cluster, but that is for next year. I am looking to take advantage of this car generation getting older and there being more and more parts out there.

Regarding the parking brake recall, this is being replaced,

I fear my brake booster is failing, which might be causing the gas mileage, but I am not certain yet.

Regarding gas mileage