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O2 sensor malfunctioning
MAF sensor dirty or malfunctioning
check for vacuum and exhaust leaks
PCV system malfunctioning / valve stuck open
check fuel pressure, weak fuel pump / clogged filter

If there is nothing immediately apparent you might want to have it checked using a professional type diagnostic machine. Its easier than throwing parts at it.
 

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Demon Spawn
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758 Posts
O2 sensor malfunctioning
MAF sensor dirty or malfunctioning
check for vacuum and exhaust leaks
PCV system malfunctioning / valve stuck open
check fuel pressure, weak fuel pump / clogged filter

If there is nothing immediately apparent you might want to have it checked using a professional type diagnostic machine. Its easier than throwing parts at it.
he just about nailed it, damn arathol your trigger is quicker than mine recently.

to easily check for vacuum leaks, outside of the obvious ripped or disconnected hose is to get you a spray bottle with water/dish soap solution and spray it on the hoses while the car is running any spot you see bubbles has a vacuum leak, flammable carb or throttle body cleaner will work too (just gotta be more careful since it is flammable and your under a car with the engine on) if you do this method instead of bubbles if you hit a vacuum leak the engine will rev. another thing I would check while under there is the air intake hose, check the ribbed part well for any minor cracks this can throw lean codes as it is allowing in unmetered air behind where the MAF sensor picks up the air volume, so the fuel mix gets thrown off but the car wont know why, just that the readings its getting are correct from the sensors but it has more air in the mix than it should. these boot do wear out, it is rare but on these cars the motor mounts failing is a big issue and when this happens it allows the engine to buck around under the hood when your driving moving and stretching everything and one of the first things to go is usually that air boot as its rubber and rips if it gets moved around violently a lot like a bucking engine does.

check the mounts while your at this too. one is extremely easy to check, its the passenger side mount it is right under the coolant tank, look at the rubber if it looks like a deflated tire or you poke it and can see rips or the fluid inside the mount its bad or if you see what looks like oil leaking under that mount beside the engine that mount is bad, if you find that mount bad replace it and the drivers rear at the same time (at minimum) the other 2 may not need changing (I had an 08 and 11 3 both needed these 2 but the other 2 where fine still) because as the passenger side one wears out it puts all the strain on the driver side one and causes it to wear out faster. the other 2 mounts are not really motor mounts but tranny mounts (2 on the auto 1 on the manual non speed cars) both the engine mounts can easily be changed in the drive way with jacks jack stands a block of wood and hand tools. the passenger mount is called mount #3 the drivers is mount #1. another way to check for bad mounts is to start the car and have a friend sit in the driver seat watch the engine as it idles a minute note any normal movement (as they will move slightly) then have him give it gas, hard to 2500 or so rpm is usually sufficient, and watch the engine you will see it lift up and set back down as he lets off the gas if you see this the mounts need changing other symptoms include leaking fluid or stains near the passenger mount (liquid filled other 4 are solid) hard or late shifting in autos and noticeably harder time going from gear to gear (especially 1-2 or 2-3) in manuals.

this video shows you what bad mounts look like:
'

this shows where the mounts are (note mount 1 is UNDER the car and can be accessed back near the drivers side firewall. and no 4 is on top of the transmission under the battery tray, #2 isn't shown here but it is a big rectangular plate under the tranny with a big piece of square rubber between the plate and a cross member that it bolts to)

https://www.google.com/search?rls=c...CYZ3k#imgrc=6cewuyPgMNRLSM:&spf=1512163456739


why did I just go into all that you might be wondering? well these mounts are a known issue on these models 13 and back non skyactiv. and when they fail they start causing all kinds of other issues possibly one like you are having where you are inexplicably throwing a lean code.

outside of this, like arathol said its kinda a toss up after checking the vacuum hoses for leaks or one that has come loose as the lean code you have is a generic code with many many things possible to fix it.

how does the car drive? any other symptoms of a problem other than the check engine light? rough, high, or erratic idle, odd shifting out of nowhere, bucking or having the rpms drop then pick back up (engine stumble)? anything else you can give us to help diagnose will help us help you
 

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Same Problem

he just about nailed it, damn arathol your trigger is quicker than mine recently.

to easily check for vacuum leaks, outside of the obvious ripped or disconnected hose is to get you a spray bottle with water/dish soap solution and spray it on the hoses while the car is running any spot you see bubbles has a vacuum leak, flammable carb or throttle body cleaner will work too (just gotta be more careful since it is flammable and your under a car with the engine on) if you do this method instead of bubbles if you hit a vacuum leak the engine will rev. another thing I would check while under there is the air intake hose, check the ribbed part well for any minor cracks this can throw lean codes as it is allowing in unmetered air behind where the MAF sensor picks up the air volume, so the fuel mix gets thrown off but the car wont know why, just that the readings its getting are correct from the sensors but it has more air in the mix than it should. these boot do wear out, it is rare but on these cars the motor mounts failing is a big issue and when this happens it allows the engine to buck around under the hood when your driving moving and stretching everything and one of the first things to go is usually that air boot as its rubber and rips if it gets moved around violently a lot like a bucking engine does.

check the mounts while your at this too. one is extremely easy to check, its the passenger side mount it is right under the coolant tank, look at the rubber if it looks like a deflated tire or you poke it and can see rips or the fluid inside the mount its bad or if you see what looks like oil leaking under that mount beside the engine that mount is bad, if you find that mount bad replace it and the drivers rear at the same time (at minimum) the other 2 may not need changing (I had an 08 and 11 3 both needed these 2 but the other 2 where fine still) because as the passenger side one wears out it puts all the strain on the driver side one and causes it to wear out faster. the other 2 mounts are not really motor mounts but tranny mounts (2 on the auto 1 on the manual non speed cars) both the engine mounts can easily be changed in the drive way with jacks jack stands a block of wood and hand tools. the passenger mount is called mount #3 the drivers is mount #1. another way to check for bad mounts is to start the car and have a friend sit in the driver seat watch the engine as it idles a minute note any normal movement (as they will move slightly) then have him give it gas, hard to 2500 or so rpm is usually sufficient, and watch the engine you will see it lift up and set back down as he lets off the gas if you see this the mounts need changing other symptoms include leaking fluid or stains near the passenger mount (liquid filled other 4 are solid) hard or late shifting in autos and noticeably harder time going from gear to gear (especially 1-2 or 2-3) in manuals.

this video shows you what bad mounts look like:

this shows where the mounts are (note mount 1 is UNDER the car and can be accessed back near the drivers side firewall. and no 4 is on top of the transmission under the battery tray, #2 isn't shown here but it is a big rectangular plate under the tranny with a big piece of square rubber between the plate and a cross member that it bolts to)


why did I just go into all that you might be wondering? well these mounts are a known issue on these models 13 and back non skyactiv. and when they fail they start causing all kinds of other issues possibly one like you are having where you are inexplicably throwing a lean code.

outside of this, like arathol said its kinda a toss up after checking the vacuum hoses for leaks or one that has come loose as the lean code you have is a generic code with many many things possible to fix it.

how does the car drive? any other symptoms of a problem other than the check engine light? rough, high, or erratic idle, odd shifting out of nowhere, bucking or having the rpms drop then pick back up (engine stumble)? anything else you can give us to help diagnose will help us help you
I’m having the same problem as OP. Wondering if you could help me out since you seem to know a lot on the issue.
My car is experiencing all of the symptoms you described when the check engine light is on. I have DTC codes P0171 and P0101 activated. I already tried cleaning out my MAF sensor which remedied the problem for a day until the light came back on the next day.

I have a 2012 Mazda 3 (2.0 liter engine)
 

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Demon Spawn
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I’m having the same problem as OP. Wondering if you could help me out since you seem to know a lot on the issue.
My car is experiencing all of the symptoms you described when the check engine light is on. I have DTC codes P0171 and P0101 activated. I already tried cleaning out my MAF sensor which remedied the problem for a day until the light came back on the next day.

I have a 2012 Mazda 3 (2.0 liter engine)
if you have those codes p0171 is bank 1 lean (inlines only have 1 bank) and p0101 is maf sensor failure, cleaning the maf rarely does anything when its actually throwing the code. you most likely need a maf sensor, but check the intake boot (tube the maf sits in and goes from filter box to engine) for rips or cracks, especially between the ribs. before buying the maf, it may just need a a $30 intake tube. if you are getting engine bucking especially while at idle or warming up that means you are either not getting enough air or enough gas into the combustion chamber i would lean towards a bad maf sensor. there are ways to test but cannot recall which wires to tap to read with a voltmeter on the 3.
 

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I'm getting a lean condition code with my 2004 Mazda 3, except I'm getting some bucking with the engine sometimes right before it shifts into 3rd gear. I had another check engine light a couple years ago that had to do with the fuel filter but was told not to worry about it. Aside from what's already been suggested, is there anything else you think I should do? All evidence points towards the fuel system.
 

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O2 sensor malfunctioning
MAF sensor dirty or malfunctioning
check for vacuum and exhaust leaks
PCV system malfunctioning / valve stuck open
check fuel pressure, weak fuel pump / clogged filter

If there is nothing immediately apparent you might want to have it checked using a professional type diagnostic machine. Its easier than throwing parts at it.
 

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2012 Mazda 3 GX MT5 non sky
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1,495 Posts
More food for though around the P0171 Lean Fuel trim code.
Which basically means after the MAF the Main O2 sensor (In the headers) is seeing
too much O2 in the burn or also means not enough fuel reaching / combusting.

I just had the same happen a few weeks ago - P0171
I also recently replaced main O2 sensor ( + down stream O2 also new from last year) and replaced with a NGK MAF
not the one from Mazda parts (which is also made by NGK) and after that it threw a a P0171 code.
I did previously resolve a exhaust leak codes downstream after the Cat.

Fuel pump (OEM 2012) and Fuel filter is 4 years old , also to consider is fouled injectors /reduced flow
which would mess A/F mixture. I have been running Lucas Injector/ fuel conditioner. On the 3 rd tank of gas with lucas.
Changed the MAF back to a cleaned Old (2012) OEM MAF / cleared codes and now no codes for 2 days.

Now i am wondering if the New NGK MAF is the issue (doubt it) but i will swap it in this weekend and see
if it throw the same code. But it has been cool weather and maybe that keeping the fuel pump from heating up
and losing pump pressure. Or the Lucas stuff is cleaning and ungumming up the gas for better flow

Another item to look at is the fuel pressure regulator. I think it sits next to the fuel pump. Fuel filter is also on my mind.
Now im also wondering if a covid/idle car with gas with ethanol is gumming up the causing to run slower.
 

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2012 Mazda 3 GX MT5 non sky
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Surprise surprise P0171 is back
Putting old OEM MAF did not work

Im going to probably have to do a smoke test
 

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Mazda 3 GT 2010
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Your fuel system is clogged up. Did you ever clean the injectors with a good Fuel injector cleaner. Maybe even Lacquer Thinner in the fuel.
Ethanol clogs up the injectors.Try cleaning your injectors, or take it to a shop to have them cleaned.

I usually put Dr.Detroit IX-40 ( DRDETROIT ) in my gas tank every 4 months, and I stopped putting Regular 87 Gas with 10% ethanol. Since I stopped, my 3 runs way better, smoother and better fuel mileage.
 

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Your fuel system is clogged up. Did you ever clean the injectors with a good Fuel injector cleaner. Maybe even Lacquer Thinner in the fuel.
Ethanol clogs up the injectors.Try cleaning your injectors, or take it to a shop to have them cleaned.

I usually put Dr.Detroit IX-40 ( DRDETROIT ) in my gas tank every 4 months, and I stopped putting Regular 87 Gas with 10% ethanol. Since I stopped, my 3 runs way better, smoother and better fuel mileage.
I agree
i only run 91 shell no ethanol
Have beenusing Liqui Moly Fuel system cleaner and lucas injector fuel conditioner
I dont think its the the injectors .. they are fairly new 50Kkms out of 270k kms on th car now.
Im leaning towards , extra air from header gasket , Intake manifold gasket or vacuum hoses leak or the fuel filter is slowing as it heats up.
Fuel filter is 4 years old (going to change that soon.
Otherwise she runs super fine ...
 

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2012 Mazda 3 GX MT5 non sky
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Put in Old MAF and got a CEL that is not the P0171
(P0300 cold start misfire - normal with the cams i have installed and can be turned off in the tune)
Maybe all that fuel cleaner and Lucas injector cleaner in 3+ tanks of gas helped.
As a double check on that i swapped out the old OEM MAF for the new NGK MAF
Lets see what Wednesday brings.
 

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After swapping the new NGK MAF for the old OEM MAzda MAF
I picked up the P0171 again.
So i decided to do another Fuel system cleaner (Liqui Moly)
Cleared the code and no more P0171 after 3 days
The Liqui Moly Fuel system cleaner is still in the tank (1/2 tank remains)
 

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2012 Mazda 3 GX MT5 non sky
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August 1 tracked the car all day - ran like new
Im thinking to do another Liqui Moly Fuel system cleaner at the end of August
It maybe over kill but i've had past issues with fouled injector because or bad fuel or
car staying idle too long
 

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2012 Mazda 3i GT
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Hey, I’ve got a 2012 3 and I recently purchased an injen short ram intake system and after intalling it and idling it to get the MAF adjusted, the car would run great for around 3-5 days and then get sloppy and I’d get a CEL with P0171 code thrown. I haven’t done the vacuum leak soap test but I did change the MAF since It was recommended that I change it at 150k miles. Have any suggestions that don’t involve putting the stock box back on?
 

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Hey, I’ve got a 2012 3 and I recently purchased an injen short ram intake system and after intalling it and idling it to get the MAF adjusted, the car would run great for around 3-5 days and then get sloppy and I’d get a CEL with P0171 code thrown. I haven’t done the vacuum leak soap test but I did change the MAF since It was recommended that I change it at 150k miles. Have any suggestions that don’t involve putting the stock box back on?
The lean code will usually reflect shortage of fuel vs airflow. because of
less flow through injectors, fuel pressure regulator, fuel pump, clogged fuel filter
or
Air is getting in after the MAF, like you mentioned -vacuum leaks from hoses valve cover or disconnected hose on the PCV system.
MAF should be able to regulate the extra air from the injen

Least expensive solution a fuel system cleaner .
 

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The lean code will usually reflect shortage of fuel vs airflow. because of
less flow through injectors, fuel pressure regulator, fuel pump, clogged fuel filter
or
Air is getting in after the MAF, like you mentioned -vacuum leaks from hoses valve cover or disconnected hose on the PCV system.
MAF should be able to regulate the extra air from the injen

Least expensive solution a fuel system cleaner .
A lean code after installing a CAI generally means either its not installed correctly and is leaking somewhere downstream from the MAF or the intake tubing is not sized correctly.
If its sucking in air downstream from the MAF, that air is not being read by the sensor. The sensor will only provide information on the air that flows past it, and the ECU will deliver fuel accordingly. When the leak is downstream from the MAF that will register as lean because there is only enough fuel for the air that is read by the MAF, and anything that enters the intake downstream from the MAF will lean the mixture out.
If the tubing is sized wrong the intake velocity will be wrong, so the readings from the MAF will be wrong and fuel delivery will be also wrong accordingly.
 

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2012 Mazda 3 GX MT5 non sky
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A lean code after installing a CAI generally means either its not installed correctly and is leaking somewhere downstream from the MAF or the intake tubing is not sized correctly.
If its sucking in air downstream from the MAF, that air is not being read by the sensor. The sensor will only provide information on the air that flows past it, and the ECU will deliver fuel accordingly. When the leak is downstream from the MAF that will register as lean because there is only enough fuel for the air that is read by the MAF, and anything that enters the intake downstream from the MAF will lean the mixture out.
If the tubing is sized wrong the intake velocity will be wrong, so the readings from the MAF will be wrong and fuel delivery will be also wrong accordingly.
Correct
You said it better !

Question :
MAF - measures the volume of air that passes by it.
MAF will read (up to a certain parameters) the flow as it passes by it dependent of certain tubing dimensions already determined and programmed by Mazda?
So a clogged airfilter will throw a CEL ( too rich)?
Reflashing ECU will compensate for the change in flow parameters caused by different CAI ?
 
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