High Fuel Trims?

[Sam9978]

First-time poster here! I just bought a 2016 CX-5 Touring with around 107K miles. It was supposed to be for my daughter as her car got totalled but she wasn't too keen on it so I'm going to use it as my Toyota has 230K miles on it and I want to get rid of it before it starts to require expensive repairs. The previous owner took very good care of the Cx-5 and did all the oil changes and other maintenance items regularly and on time.

Recent items done (not at a dealer): 10/13/2023, 82,000 miles - Transmission service, Brake fluid flush, Coolant Flush, Fuel Injection Service, Brakes & Rotors (all)

I've only had it for a week but I noticed today that the fuel trim is quite high, 8.5% for LT and 3% for ST. It doesn't go up much when driving, just a few percentage points at most and sometimes ST will be around zero depending on the environment, driving style, etc.

I asked the dealer service department about the fuel trim and they weren't at all concerned. I asked if carbon buildup on the intake valves could do it (seeing as it's close to 100K miles) but they didn't think it would affect the fuel trim, at least not more than a percentage point or two. I've heard conflicting things about this. Any thoughts (see related second question below)?

So, all that to say, I was wondering if anyone has any ideas I can try myself to see if I can get this a little closer to normal before taking it to a mechanic. Or am I worried about nothing?

Another question: Do I need to get the intake valves and fuel injectors cleaned seeing as it's over 100K miles? I've heard Mazda is better about this than other makes but from what I understand this is always going to be an issue with GDI engines (except maybe the dual-injection engines). The previous owner did fairly decent length trips in it but I will be doing much shorter trips. Is it worth getting the $200 fuel induction service (the dealer quoted $199.99) or should I be looking at finding a place that does walnut blasting?

I've always been a Toyota guy but I'm looking forward to my first Mazda as I've heard great things about them.

I appreciate any help or advice. Thank you in advance!

 

[skid00skid0]

Fuel Trim: What the numbers mean

Discover how fuel trim affects your vehicle's air-fuel mixture and how understanding it can help diagnose engine issues.

ricksfreeautorepairadvice.com

 

[Lazy2.5]

Prompted me to take a look at the fuel trims again on my 91 octane tuned Skyactiv 2.5 . I am seeing LTFT's ranging between -1 to 5%, and STFT as high as 15% during hard acceleration. I recently hit 200,000 miles, and the drivetrain is well maintained.

However, when driving along normally (Steady state cruising at 1500rpm give or take, which is pretty much what this drivetrain wants to be doing), trims are very close to zero.

Considering that you are achieving 13mpg according to your screen read-out thing, I would suggest to lay off the throttle for a tank or two and see how your LT, ST - FT's improve.

 

[200955]

I'm not totaly sure but I believe what you are getting are "averages" on those readouts. What you really want to be looking at is current "real time" data under varying loads. I cannot believe that 13.95 miles per gallon is anywhere near accurate. As for the valve cleaning, you really need to remove the intake and do an inspection to make a determination. Have you any legacy codes or current ones showing up on your diagnostic scanner?

 

[Sam9978]

Prompted me to take a look at the fuel trims again on my 91 octane tuned Skyactiv 2.5 . I am seeing LTFT's ranging between -1 to 5%, and STFT as high as 15% during hard acceleration. I recently hit 200,000 miles, and the drivetrain is well maintained.

However, when driving along normally (Steady state cruising at 1500rpm give or take, which is pretty much what this drivetrain wants to be doing), trims are very close to zero.

Considering that you are achieving 13mpg according to your screen read-out thing, I would suggest to lay off the throttle for a tank or two and see how your LT, ST - FT's improve.

Thank you.

I think that 13mpg average was because all I did was drive around the block to see if the trim levels went up or down under load. I'm generally a very good driver and I consistently get excellent mileage in my old 4cyl, 2.7ltr, 2009 Highlander with 230,000 miles on it.

On Saturday I reset the MPG average and went to the dealer and a few other small stops on the way -- around 30 miles round trip with roughly 70% highway driving -- and I got an average of 24.5 mpg with the highest average being 26.5%. I have noticed that short trips in town get pretty bad average of between 15 to 19 mpg.

I mainly wanted to see if anyone had any ideas of where to start looking to try to track this down. I guess I'll monitor the fuel trims more closely for the next week or so and see what they do under normal driving conditions and then circle back to this.

 

[Lazy2.5]

Thank you.

I think that 13mpg average was because all I did was drive around the block to see if the trim levels went up or down under load. I'm generally a very good driver and I consistently get excellent mileage in my old 4cyl, 2.7ltr, 2009 Highlander with 230,000 miles on it.

On Saturday I reset the MPG average and went to the dealer and a few other small stops on the way -- around 30 miles round trip with roughly 70% highway driving -- and I got an average of 24.5 mpg with the highest average being 26.5%. I have noticed that short trips in town get pretty bad average of between 15 to 19 mpg.

I mainly wanted to see if anyone had any ideas of where to start looking to try to track this down. I guess I'll monitor the fuel trims more closely for the next week or so and see what they do under normal driving conditions and then circle back to this.

Got it! It's normal for fuel trims to be high, initially, after a quick drive around the block at low speed.

15mpg sounds unrealistic around town. Try hand calculating your mileage. If you confirm that readout is accurate, then perhaps I would trust those fuel trims too.

But even if it were accurate, a short drive at low speed will not provide you with any definitive evidence.

So, determine that the information you're getting is accurate over a longer period of driving, such as 1-2 full tanks, and then let's assess what your fuel trims likely are in reality, and then see if anything needs addressing.

If your fuel economy really is that low, then yes. But let's see first .

 

[Sam9978]

I'm not totaly sure but I believe what you are getting are "averages" on those readouts. What you really want to be looking at is current "real time" data under varying loads. I cannot believe that 13.95 miles per gallon is anywhere near accurate. As for the valve cleaning, you really need to remove the intake and do an inspection to make a determination. Have you any legacy codes or current ones showing up on your diagnostic scanner?

Thank you for the reply.

Yes, the mpg is average, and the fuel trims are real-time. I need to configure the car scanner app to show real-time and see what it shows me. I wish the Mazda system showed me real-time but it only shows an all-time average on the dash and trip average in the infotainment system. I believe the 13mpg is because I did a very short trip around the block to see how the fuel trims would react.

There are no legacy or current codes showing and the MIL was last reset around 25,000 miles ago which coincides with the time the battery was replaced.

I wish I could remove the intake and do an inspection but I don't have a place to do that. I was considering trying to find a place to get it walnut blasted but a Google search doesn't return any places in my area which I find odd.

I may just do the dealer fuel induction service and see what that does if anything and take it from there.

 

[Sam9978]

Got it! It's normal for fuel trims to be high, initially, after a quick drive around the block at low speed.

15mpg sounds unrealistic around town. Try hand calculating your mileage. If you confirm that readout is accurate, then perhaps I would trust those fuel trims too.

But even if it were accurate, a short drive at low speed will not provide you with any definitive evidence.

So, determine that the information you're getting is accurate over a longer period of driving, such as 1-2 full tanks, and then let's assess what your fuel trims likely are in reality, and then see if anything needs addressing.

If your fuel economy really is that low, then yes. But let's see first .

Sounds good, thank you.

Do you feel it's worth the extra expense of filling up with 91 octane fuel? I have no idea what the previous owner used but I'd guess just regular.

 

[Sam9978]

Fuel Trim: What the numbers mean

Discover how fuel trim affects your vehicle's air-fuel mixture and how understanding it can help diagnose engine issues.

ricksfreeautorepairadvice.com

Very interesting, thank you.

My situation seems to fall under Positive/Any on the interpretation table. I'll try cleaning the MAF and MAP sensors (MAP is ok to clean with MAF cleaning fluid, right? I've seen conflicting opinions) and see if that helps any. If not is there a way to test the fuel system and is it something I could do? ...I don't have a garage and only basic tools.

A friend who's fairly decent with cars suggested I try a few tanks of ethanol free gas and add some fuel system cleaners. Do you think that's worth trying?

 

[Lazy2.5]

A friend who's fairly decent with cars suggested I try a few tanks of ethanol free gas and add some fuel system cleaners. Do you think that's worth trying?

Absolutely, yes. To answer your question, I recommend using whichever fuel is ethanol free.

Please take and upload a pic of your exhaust tips. It would provide good insight as to your engines conditions and how it's been used over the years.

 

[Sam9978]

Here you go. Pictures are named as to which side they are. They look pretty decent to my untrained eye. I also included a picture of my son's 2010 Corolla with the same amount of miles just for interest.

 

[Lazy2.5]

Here you go. Pictures are named as to which side they are. They look pretty decent to my untrained eye. I also included a picture of my son's 2010 Corolla with the same amount of miles just for interest.

Your CX-5 tips looking clean. I have no doubt that this engine burns little to no oil, and the previous owner likely treated the car right (at least as far as throttle input goes.) excess run off onto the chrome tip (such as with your corolla) Indicates a potential issue or heavy throttle application with the Skyactiv's.

 

[Sam9978]

Thank you. I'll keep an eye on things for a few weeks and also try a few tanks of 91 or ethanol-free fuel with Techron or Redline and then report back on my findings.

Thank you for your time (and everyone else who answered) it is much appreciated!

 

[Lazy2.5]

Here you go. Pictures are named as to which side they are. They look pretty decent to my untrained eye. I also included a picture of my son's 2010 Corolla with the same amount of miles just for interest.

Here's mine for reference. You should not see black liquid running off onto the chrome tip with these Skyactiv's. If you do, suspect worn internals such as seals, rich mixture, aggressive or hard use.

 

[Sam9978]

Good to know, thank you.

Out of interest, do you think it's worth replacing the AFR (upstream O2) sensor? I've heard it's good to replace around 100K miles. The Denso OEM replacement is $100 before tax and shipping.

 

[Lazy2.5]

Good to know, thank you.

Out of interest, do you think it's worth replacing the AFR (upstream O2) sensor? I've heard it's good to replace around 100K miles. The Denso OEM replacement is $100 before tax and shipping.

Wouldn't hurt, but there's no need to if you don't have any code. It's likely functioning as intended.

I would monitor MPG and go from there.

 

[Jmaz]

Thank you for the reply.

Yes, the mpg is average, and the fuel trims are real-time. I need to configure the car scanner app to show real-time and see what it shows me. I wish the Mazda system showed me real-time but it only shows an all-time average on the dash and trip average in the infotainment system. I believe the 13mpg is because I did a very short trip around the block to see how the fuel trims would react.

There are no legacy or current codes showing and the MIL was last reset around 25,000 miles ago which coincides with the time the battery was replaced.

I wish I could remove the intake and do an inspection but I don't have a place to do that. I was considering trying to find a place to get it walnut blasted but a Google search doesn't return any places in my area which I find odd.

I may just do the dealer fuel induction service and see what that does if anything and take it from there.

If it were me, I'd hold off on the dealer induction service. It's liable to be a can of some cleaner fed into the intake manifold for the princely sum of ~$200 as you mentioned. Money better spent on walnut blast /manual cleaning if that really is needed yet. I'd run fuel system cleaner and some higher octane gas for a tank or two.
Regarding LTFT, a former neighbor mechanic once told me that it was common to see 8-10% (or even a little more) on cars going through his shop that had a lot of local/short trip driving. To me it made sense, given my understanding if correct, that many cold starts and short trips with the accompanying richer mixture (more fuel added -- STFT) contributed to larger LTFT which I understand is derived from STFT data.
Lastly, there is a thread on this forum where "high miler" CX-5 owners report their mileage. I wonder how many 150-200-250 K milers ever had the intake valves cleaned.

 

[Sam9978]

If it were me, I'd hold off on the dealer induction service. It's liable to be a can of some cleaner fed into the intake manifold for the princely sum of ~$200 as you mentioned. Money better spent on walnut blast /manual cleaning if that really is needed yet. I'd run fuel system cleaner and some higher octane gas for a tank or two.
Regarding LTFT, a former neighbor mechanic once told me that it was common to see 8-10% (or even a little more) on cars going through his shop that had a lot of local/short trip driving. To me it made sense, given my understanding if correct, that many cold starts and short trips with the accompanying richer mixture (more fuel added -- STFT) contributed to larger LTFT which I understand is derived from STFT data.
Lastly, there is a thread on this forum where "high miler" CX-5 owners report their mileage. I wonder how many 150-200-250 K milers ever had the intake valves cleaned.

Very interesting, thank you.

I have a friend who is pretty good with cars (he does all his own work unless it requires a lift or some specialized knowledge or tool) and he said he might have time to help me clean the throttle body and take off the intake manifold and check out the valves and, if he has time, possibly clean them as well.

In the meantime, I'll run a few tanks of higher octane gas with some cleaners and see if that helps at all.

 

[Sam9978]

Wouldn't hurt, but there's no need to if you don't have any code. It's likely functioning as intended.

I would monitor MPG and go from there.

Just did a 4 mile trip (2 miles each way) and here are the fuel trims. I was going to video them but I got a call while driving so I couldn't.

The averages from my scan tool are out of wack, as you can see, I probably need to calibrate it a bit with the correct car info, etc.

 

[Jmaz]

Very interesting, thank you.

I have a friend who is pretty good with cars (he does all his own work unless it requires a lift or some specialized knowledge or tool) and he said he might have time to help me clean the throttle body and take off the intake manifold and check out the valves and, if he has time, possibly clean them as well.

In the meantime, I'll run a few tanks of higher octane gas with some cleaners and see if that helps at all.

Yea, that sounds good. At least the throttle body cleaning isn't too difficult. Just a note here about that. I replaced the throttle body gasket when I cleaned mine. The OEM Mazda was priced around $20. A Beck-Arnley brand I bought online cost $3-$4 plus shipping. It was identical, even made in Japan as the original. But, the original could have been reinstalled since it was still so resilient-- silicone gasket.
I think you'd do well to address the items that make it run smooth and contribute to reliability and not get too freaked out about LTFT values. As the old saying goes-- "Drive it".
And speaking of running smooth, have the spark plugs been changed? Mazda recommends plug change at 75K miles for my CX-5. I changed them at 71K a few months ago and am still loving the improvement in smoothness and stronger pulling out. Not huge but nice improvement.