2019 CX-5 with 23,000 miles. Recently mileage has decreased. Dealer cleaned injectors and intake system, no improvement. Seems to have slight hesitation upon acceleration. We live in Tucson area where its been really hot. I am of the opinion this may be due to hot weather, where the reduced density means less air thru the engine and the computer adds fuel to maintain power. Any suggestions?

 

Need to know the difference in MPG and when it started. Month?
How did they clean the injectors. There are several service procedures not all the same and wondering why they did this as a first option?
At 23k miles you engine just got broken in completely. What gas octane are you using and what brand?
Are you using your AC more now than before you noticed a drop in your MPG
I would not speculate on the ECU operation as you are theorizing the Skyactiv ECU programing compensates depending on a few factors which I also need to know to theorize about what your ECU may be doing. But it is not as drastic as you may think and depending on your answers may and likely not be what you are thinking for the fueling, timing etc. regarding the ECU. Example my days here are currently in the upper hundreds with record triple digits over 112F-114f ambients my MPG is the same as normal for this time of year here.

 

The car is 2020, not 2019. Had it exactly 3 years. It is wife's. I seldom drive it. MPG complaint came from her. I think she watches the MPG indicator, She also has a phone app where she tracks fill-ups. Woe to me if I forget to report. I've attached 2 plots of MPG, data from her mileage app, the past year to date and the past six months to date. The bars over 30 mpg are trips to/from Las Vegas. If you exclude those I am not sure I see much decrease. The one recent bar below 25 mpg, no idea about that. Most driving is around town. The two most recent mpg bars are a trip to/from Phoenix, after injector service.


I have no idea what cleaning process the dealer used. Service advisor suggested it after I asked about mileage decrease. Paperwork says "Perform complete fuel injector system service. Routine maintenance. Service cleans all fuel injectors, intake manifold, ports, intake valves and combustion chamber as applicable". Cost was $132. As far as gas we buy only Top Tier 87 octane, usually Quick Trip but often Chevron or other top tier brand. Rarely not top tier. A/C is on whenever the car is on. Just what we do.

My temperature speculation was simply that. Lower density means less air thru engine. I would assume ECM reduces fuel flow to maintain proper mixture. That would be less power, but old Lead Foot might compensate by stomping harder on the pedal. On the other hand mpg bars from last winter and spring seem about the same is the recent ones.

 

A proper fuel injection cleaning by itself is about 1.50 hours labor. The average coast is 150.oo labor only. just an FYI.

An intake cleaning depending on the method and if it did include the intake port is a few hundred dollars.
I have no idea how they claimed clearing the valves and combustion chambers..... UNLESS the can of F/I cleaner claimed cleaned all of that LOL

The ECU (in your Mazda) first job is to maintain all the engines function for the lowest emissions. Not like a few decades ago. In which case your air density info would be spot on!

Running the AC all the time and or contact use of the defrost which actuates the AC compressor does effect mpg.


I don't really see anything alarming in those graphs that would have me looking for the preverbal needle in the haystack for MPG loss? IMHO it has to do with driving habits! The best advice but be clear it is not easy and NOT SAFE for many drivers to use your instant MPG monitor and retrain your driving habits?

You may also consider changing your spark plug. Please don't tell me how many miles are on them if there are more then 5k miles and close or over 10k then they are not at the best performance. An ARGUMENT I have often on forums yet can easily be proven byway of an oscilloscope!


ASE

 

I was told by an old timer mechanic to never use the cheapest lowest octane gas available, because even though it is advertised as 87 octane it may not be. It could be 86 or less. Modern cars have sensors to adjust for low octane, and the result is less power/ mileage. He recommended using mid grade, because its better to have more octane than not enough.

 

Well I hate to use the word WRONG but he was. At least in the USA that would be a Federal violation and the station selling the gas would be fined and or closed either for 30 days or permanently. You can look up all of this on the internet about fuel retail selling in the USA.
As for the second part well it is somewhat accurate.... the sensors are reporting data/information to teh ECU which makes "small" adjustments in engine operations that gas octane can affect to some degree. And yes depending on a few dozen varibles can and often does affect the MPG both in a positive and negative way. My advice is as the engine gains miles to start making the shift to using high grades of fuel and octane.

It would also not be a bad idea to pour in a fuel injection cleaner every so often like Chevron, STP or even Clean Power. While I can't say for absolute certainty that they help I still use them myself and many time will recommend them to customers that are using fuels from none top tier retailers like Short Stop, 7/11, costco, and a few others of this tyep. . Based on fuel formulas the top 2 fuels in the USA are Chevron and Shell with Union 76 following up behind them.
Doing specific gravity sample testing confirms this so it is not an opinion.

 

We generally use Top Tier fuel, although occasionally a fill-up from a a non-Top Tier station. Mazda doesn't recommend Top Tier fuel. However I thought the Top Tier additive was basically Techron. So it would be better than non-Top Tier fuel. We buy Quik Trip (QT) mostly because the station is close to home. When on the road we buy Chevron or Shell. Is there much difference between fuel brands, for the same octane rating?

I was under the impression that buying higher octane gas can damage the engine over time, since the ECU will advance the timing slightly beyond what the engine is designed for. Am I wrong?

 

Mazda only show the recommended octane to use. The octane value equates to their power rating claims
Where ever you got the information about higher octane fuel is badly incorrect. At least since OBDII (1995) ECUs and a few CANbus ECU programing from about 2002 and newer. And predominately with any MAZDA Skyactiv engine.
The sensors on most platforms report to the ECU and the ECU will make adjustments for the octane values used. To a point after that you would likely get a DTC for a PID having trouble. Top Tier fuels like I mentioned their formulas include additives to help the engine and the fuel system remain cleaner than those of other fuel suppliers. Those special additives in the fuel formulas are part of the extra cost per gallon you pay. Occasionally using the lower brand fuels may not harm the engine in short term but the effect will show after several years or after 80-100k miles. At some point your MPG will also start a slow decline at first you may not notice it or thinks it spark plugs or other area of fueling and induction needing servicing.
I can easily see when anyone is using lower octane fuels by examine the spark plug with a magnifying glass. You can see the hot spots because of incomplete combustion under a load. Also, if I go into the knock sensor values of the ECU, we can clearly see the ECU struggling at times to keep the engine running at its best level under load conditions with the use of lower octane fuels.

Anyone can test this by use of a Diagnostic scanner connected to the OBDII diagnostic connector direct only not blue tooth . Go to view knock sensor and timing values and watch the different between lower octane fuels and higher octane fuels.

To be clear so that no one misunderstand me I never said not to use lower octane fuels I only posted what those lower octane fuel will do.

 

I picked up a bottle of Techron. Chevron says to use only once a season or else no more often than twice between oil changes. Does that sound right?

In a related topic the Trip A and Trip B tripmeter average fuel mileages seldom agree. Sometimes they differ by a tenth or two. Sometimes by many mpg. What is up with that? Also, I've noticed the remaining miles number sit still for many miles, and then suddenly jump to a lower value. I would expect a slowly decreasing value, not steady and then sudden jump.