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CEL - System Too Lean (P0171) ) 07 Corolla


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HighwayBob
New User

Dec 21, 2020, 9:47 PM

Post #1 of 8 (2109 views)
CEL - System Too Lean (P0171) ) 07 Corolla Sign In

I have a 2007 Toyota Corolla with about 106k miles on the odometer. My CEL came on a couple of weeks ago and when I connected my BlueDriver I saw code P0171. Captured the freeze frame and ran with the BlueDriver in live mode. My short term fuel trim was around 20% and the long term fuel trim was around 30%.

I found a pretty good YouTube video and used it as the basis of my troubleshooting.

I just did the basics, checked for pirate air through bad vacuum lines, etc. and I checked the air tube leading to the engine from the air filter housing. Everything looked okay. During this I discovered I had a dirty air filter and replaced it, but that shouldn't come into play here.

I removed the Mass Airflow Sensor to examine it, and it looked very clean, so I re-installed it. When I started up the engine again the fuel trims were dramatically different and what I would consider "normal". Short term was hovering within +/- 5 of 0 and long term was about 7. I drove it for a while with the BlueDriver in live mode and everything looked good, so I cleared the codes and reset the light. I thought I had taken care of it somehow.

About a week later the CEL came back on. Same code, same numbers just about. Again, I got the freeze frame and it looked about the same as first time (I have all these numbers if needed).

Well, this time I did nothing. I drove about 15 minutes into work, again in live mode, and the fuel trim numbers started off bad but calmed down pretty quickly to a normal range. I thought maybe the issue popped up only with a cold engine and I was planning on re-checking it after it had been sitting for about 4 hours thinking I could reproduce the issue, however after letting the car get cold and then starting it up the fuel trim numbers remained normal. I cleared the code again.

So now it's been about two days since this second incident. I have kept my BlueDriver connected and in live mode every time I've driven it and again my fuel trim numbers have been normal.

Obviously there is some issue here and I'd rather not have to bring the car into my mechanic since the problem seems tough to reproduce. I was thinking about replacing the MAF sensor but am not sure that would be beneficial.

Just looking for some ideas about what the root cause is here. I am far from able to do complicated diagnostics and while I know something about cars don't consider myself very competent to do anything serious.

I also don't know what normal numbers are for things like mass airflow rate, so I'm really judging "normal" based on the fuel trim numbers.

Any input would be appreciated!


(This post was edited by HighwayBob on Dec 21, 2020, 9:49 PM)


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Dec 22, 2020, 6:47 AM

Post #2 of 8 (2084 views)
Re: CEL - System Too Lean (P0171) ) 07 Corolla Sign In

Your methods of "checking" things is a bit insufficient.

You can't "look at" a MAF and know it's clean. You're looking for microscopic dirt on a tiny thin wire.

When you're looking for a vacuum leak you really need a smoke tester.

You also need to monitor the fuel pressure "during the failure" Low pressure will do that also.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



HighwayBob
New User

Dec 22, 2020, 7:56 AM

Post #3 of 8 (2071 views)
Re: CEL - System Too Lean (P0171) ) 07 Corolla Sign In

I was thinking of trying one more thing - cleaning the MAF sensor. It's relatively cheap and easy to do. If the issue recurs it's time for my shop to have a look at it.

Thanks for your help.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Dec 22, 2020, 7:58 AM

Post #4 of 8 (2067 views)
Re: CEL - System Too Lean (P0171) ) 07 Corolla Sign In

Good idea. By a cleaner specifically made for that purpose.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



HighwayBob
New User

Dec 22, 2020, 9:30 AM

Post #5 of 8 (2054 views)
Re: CEL - System Too Lean (P0171) ) 07 Corolla Sign In

Two more quick questions:

- should the short term fuel trim be about the same regardless of whether the engine is hot or cold? I just checked after the car sat all night and the initial STFT was around 10 and gradually came down to around 0 as the engine warmed up

- I'm wondering if my MAF rate numbers are off. From idle to about 1000 rpm the rate is about 2.5 gm/sec and at 2500 rpm it only goes up to about 7.5 gm/sec. I (for whatever reason) just expected the rate to be higher than 7.5

Thanks again for your input. It's appreciated.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Dec 22, 2020, 9:36 AM

Post #6 of 8 (2049 views)
Re: CEL - System Too Lean (P0171) ) 07 Corolla Sign In

Forget about both of those things.

STFT is a constant, instantaneous fluctuation. The only thing that matters to you is the LTFT.

You can't use MAF readings because they also constantly change with engine changes. The computer has the ability to analyze those readings but the naked eye does not have the ability to see anything beyond steady idle.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



HighwayBob
New User

Jan 8, 2021, 6:13 PM

Post #7 of 8 (1966 views)
Re: CEL - System Too Lean (P0171) ) 07 Corolla Sign In

Just some updated info...

Cleaning the MAF sensor didn't have any noticeable effect.
(yes, I used MAF sensor cleaner)

I have noticed a more specific set of circumstances that cause the issue.

Outside temp here is in the 30's and low 40's. When the car sits overnight, say, and I start it up, STFT gradually climbs from low single digits to the low 20's and LTFT starts out mid to high single digits and gradually climbs to low 30's. This occurs when idling or when driving (street driving). When the engine reaches operating temperature (180-185) the STFT drops to within +/- 5 of 0 and LTFT comes back down to high single digits, with maybe a max of 10 or so.

If I jump right on the highway, the higher throttle seems to keep both trim numbers down and there is no issue.

Any input is appreciated.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Jan 8, 2021, 6:25 PM

Post #8 of 8 (1962 views)
Re: CEL - System Too Lean (P0171) ) 07 Corolla Sign In

I'm sure you've heard the expression "A little bit of knowledge is a dangerous thing".

That is the case here. Put down your DIY code reader. It's not helping. Get back to basics. Get a smoke test done to look for vacuum leaks and test the fuel pressure.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.







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