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Mazda3 long term fuel trim high at all times #5245793 10/21/19 09:12 AM
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windeye Offline OP
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It's the 2007 Mazda3 in my signature. I am doing some data log on this car just to see what's going on with the Mazda3 after I replaced the O2 sensors on my Mazda MPV. The car has no DTC, and drives well. One thing stands out is the high LTFT. It's about 7-9% at idle, and can go to 15% at load. For those of you who monitor your Mazda3, what do you see for your LTFT? The car is a 2.0L 4-cyl engine, has one A/F sensor, and two O2 (narrow band) sensors downstream of exhaust.

I don't think high LTFT is normal, but it's probably not out of whack yet to set DTC. I had an Innova 3130 for a long time, but it's very limited in terms of graphing live data. I am getting a OBDLink MX+, will play with it once I get it on my hand.


2000 Mazda MPV LX, 202,500 miles
2007 Mazda 3 Touring, 112,800 miles
2013 Toyota Camry XLE, 100,800 miles
Re: Mazda3 long term fuel trim high at all times [Re: windeye] #5245805 10/21/19 09:25 AM
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Donald Online Content
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What is the fuel pressure from the pump? Fuel injectors need a professional cleaning so they are matched.


2015 Subaru Forester 2.5 engine/CVT
2015 Ford F250 w/Powerstroke
2016 Subaru Crosstrek CVT (wife's)
Re: Mazda3 long term fuel trim high at all times [Re: windeye] #5245810 10/21/19 09:28 AM
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WyrTwister Offline
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Any vacuum leaks ?


Wyr
God bless
Re: Mazda3 long term fuel trim high at all times [Re: Donald] #5245818 10/21/19 09:35 AM
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windeye Offline OP
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Originally Posted by Donald
What is the fuel pressure from the pump? Fuel injectors need a professional cleaning so they are matched.

I haven't done any fuel pressure test yet. It'd be messy since car does not have a Schrader value, and it's a returnless fuel system. I'll see if the new OBDLink can give me fuel rail pressure.


2000 Mazda MPV LX, 202,500 miles
2007 Mazda 3 Touring, 112,800 miles
2013 Toyota Camry XLE, 100,800 miles
Re: Mazda3 long term fuel trim high at all times [Re: WyrTwister] #5245828 10/21/19 09:43 AM
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windeye Offline OP
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Originally Posted by WyrTwister
Any vacuum leaks ?


Not that I can find. Usually, if there is a vacuum leak, the LTFT will improve under load as the manifold vacuum reduces. I ran some numbers to calculate VE at various points, VE seems to be at 80% at best, which suggests a under-reporting MAF sensor. At idle (700 rpm), it reads 2 g/s, which is good for a 2.0L engine (per rule of thumb). I don't have a known good mass air flow number at high rpm to compare to.

I took the MAF sensor out, it was not dirty. cleaned it anyway with MAF cleaner. no effect. It could be weak fuel pump or injector.


2000 Mazda MPV LX, 202,500 miles
2007 Mazda 3 Touring, 112,800 miles
2013 Toyota Camry XLE, 100,800 miles
Re: Mazda3 long term fuel trim high at all times [Re: windeye] #5245988 10/21/19 12:16 PM
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Chester11 Offline
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Not the same model - but I have the Mazda 6 with the 2.3 from that year.

I went down the same rabbit hole chasing down high fuel trims. If memory serves LT was in that range at idle as well.

You probably already did this - but the snorkel near the air box can be the source of a vacuum leak. Also the connections on the brake booster line.

I never did check fuel pressure, but continuity wise the injectors checked out ok. I gave up, replaced a bad motor mount, and called it a day.

All the Mazda specific boards are somewhat dead from this generation - so I was never able to find reliable timing advance or MAF data for comparison.

Re: Mazda3 long term fuel trim high at all times [Re: Chester11] #5246062 10/21/19 01:05 PM
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windeye Offline OP
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Originally Posted by Chester11
Not the same model - but I have the Mazda 6 with the 2.3 from that year.

I went down the same rabbit hole chasing down high fuel trims. If memory serves LT was in that range at idle as well.

You probably already did this - but the snorkel near the air box can be the source of a vacuum leak. Also the connections on the brake booster line.

I never did check fuel pressure, but continuity wise the injectors checked out ok. I gave up, replaced a bad motor mount, and called it a day.

All the Mazda specific boards are somewhat dead from this generation - so I was never able to find reliable timing advance or MAF data for comparison.

Hi, Chester, Thanks for your note. I'll check the snorkel and the brake vacuum more carefully... It seems the computer can manage this for now. I replaced the passenger side motor mount a few years ago as well. First got a unit from NAPA, looked identical to OE, but then vibration started to go very noticeable after a few months. Got an OE part, returned the NAPA one. All has been well, motor mount wise. Engine control, circuit, electronic stuff are hairy to me, but am trying to get my mind around those things.


2000 Mazda MPV LX, 202,500 miles
2007 Mazda 3 Touring, 112,800 miles
2013 Toyota Camry XLE, 100,800 miles
Re: Mazda3 long term fuel trim high at all times [Re: windeye] #5246388 10/21/19 06:42 PM
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windeye Offline OP
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Just a quick note... saw a YouTube video by Well Tech on fuel trim and MAF sensor, and a follow-up video on MAF cleaning. The host put the hot-wire elements under a 100x microscope to really see the fine dirt that's caked on the elements. MAF cleaner and brake clean (he used) can take some dirt away, but the coating is damaged, which the hosts says renders the MAF sensor to be inaccurate.

Another trick he used to see if the fuel delivery problem is the main cause: he unplugged the MAF sensor. The truck he is fixing falls back on MAP reading for fuel control, and the data were good.

I'll try that as well. The Mazda3 does have an MAP sensor to fall back on. Still learning.


2000 Mazda MPV LX, 202,500 miles
2007 Mazda 3 Touring, 112,800 miles
2013 Toyota Camry XLE, 100,800 miles
Re: Mazda3 long term fuel trim high at all times [Re: windeye] #5246410 10/21/19 06:57 PM
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Chester11 Offline
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Hope it helps - I have struggled with getting info from the Mazda folks, and the kind gurus here felt sorry for me.

Unfortunately that was my experience with the NAPA mount as well.

I forgot to mention - my scan tool is a little slow - but I did notice the A/F sensor was more "active" than I expected. Meaning that it would raise the fuel trims up high (about 15) during acceleration, and then drop negative when dropping off the throttle. (It also drops into open loop at full throttle, which I thought was kinda cool for some reason).

The factory manual (for the 6 anyway) has a test procedure where you count the peaks on the A/F sensor graph and the number of times it crosses a certain amperage. I was never confident enough in my diagnostic ability to justify replacing the sensor, but kicked it around until I saw the price.

Another common vacuum leak area is apparently the PCV hose - which is buried behind the intake manifold. (If you want a laugh - check on a youtube walk-thru on that s-show - some actually take the bumper off to replace the hose and valve - no thanks). I just shot some brake fluid back there and was pretty confident mine was ok. But if I ever see the trims go consistently nuts - that is where I am going to look first.

The system is pretty good at managing the trims - you can pull the brake booster hose right off and not set a code.

The only other oddity I noticed in the data was that the timing advance moves around quite a bit. I was never able to figure out why - or if it was a problem - but I know some guys adjusted the cam position sensor to do - well, something. It sound like BS bro-science to me, so I left it alone.

You in the part of NY where there can be a snow flurry on Halloween? (Former buffalo guy here)

Re: Mazda3 long term fuel trim high at all times [Re: windeye] #5246495 10/21/19 08:16 PM
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clinebarger Offline
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I wouldn't worry about those LTFT readings, If you had a vacuum leak....The trims will correct under heavy throttle/load.
If the STFT are staying near center.....The PCM is compensating/Has to ability too.

Data logging is a fantastic tool, But don't over analyse the data.....NO pedestrian/factory/mass produced calibration is 100% perfect & [censored] sure not perfectly matched to mass produced Injectors/MAP sensors/MAF sensors!

When you have no performance issues & no DTC's......That's a known good data log of that vehicle!


2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/U341E
2000 Toyota Avalon 1MZ-FE/A541E
Re: Mazda3 long term fuel trim high at all times [Re: clinebarger] #5246652 10/22/19 03:19 AM
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Mad_Hatter Offline
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Originally Posted by clinebarger
I wouldn't worry about those LTFT readings, If you had a vacuum leak....The trims will correct under heavy throttle/load.

But they're not correcting, the LTFT goes up at WOT when it should go down (if there was a vacuum leak) because you run a richer fuel/air mix at WOT. This tells me that the ECU is trying to compensate for something at WOT beyond normal wear and tear; either unmetered air (not likely) or less than the commanded amount of fuel or one or more sensors (MAP, MAF, A/F) is reporting incorrectly. Fwiw, he won't get a DTC until LTFT hits something like +20% but that doesn't mean there's not a problem per se (no DTC). Running excessively lean or rich can effect FE, performance, emissions and wear.

The LTFT at idle is more or less fine for a car that age. Generally anything under 8% is considered acceptable for an older vehicle. What's more problematic is the increasing LTFT at WOT. If I had to guess, I'd go with fuel delivery (low pressure or clogged injectors). But it's worth checking everything, like each sensor, before throwing money at the problem.


Last edited by Mad_Hatter; 10/22/19 03:31 AM.
Re: Mazda3 long term fuel trim high at all times [Re: windeye] #5246884 10/22/19 09:09 AM
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painfx Offline
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It would be nice if you can post up some live data. RPM, Loop Status, STFT, LTFT, Calculated Load, and A/F/Oxygen Sensor readings.

Data: one at idle, one at cruise and one at wide open throttle.

Re: Mazda3 long term fuel trim high at all times [Re: Mad_Hatter] #5247586 10/22/19 08:48 PM
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clinebarger Offline
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Originally Posted by Mad_Hatter
Originally Posted by clinebarger
I wouldn't worry about those LTFT readings, If you had a vacuum leak....The trims will correct under heavy throttle/load.

But they're not correcting, the LTFT goes up at WOT when it should go down (if there was a vacuum leak) because you run a richer fuel/air mix at WOT. This tells me that the ECU is trying to compensate for something at WOT beyond normal wear and tear; either unmetered air (not likely) or less than the commanded amount of fuel or one or more sensors (MAP, MAF, A/F) is reporting incorrectly. Fwiw, he won't get a DTC until LTFT hits something like +20% but that doesn't mean there's not a problem per se (no DTC). Running excessively lean or rich can effect FE, performance, emissions and wear.

The LTFT at idle is more or less fine for a car that age. Generally anything under 8% is considered acceptable for an older vehicle. What's more problematic is the increasing LTFT at WOT. If I had to guess, I'd go with fuel delivery (low pressure or clogged injectors). But it's worth checking everything, like each sensor, before throwing money at the problem.




But it is compensating, It's adding fuel & well within its capabilities. I think it's 21% or 22% before it sets a lean DTC.

He didn't specifically say WOT, I'm not so sure this one falls out of closed loop at WOT or has a overly rich PE like the older system did.


2001 Chevy Camaro L92/4L80E
2006 Chevy 2500HD LBZ/Allison 1000
2010 Toyota Corolla 2ZR-FE/U341E
2000 Toyota Avalon 1MZ-FE/A541E
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