P0171 Lean Bank appears and disappears

[Tundra9er]

2000 Limited automatic with about 170k

About 2 - 3 weeks ago I suddenly receive a Check Engine light. The 4Runner had been running great for a while so this came out of the blue. Took it to Auto parts store, OBD shows P0171 too Lean code. I remove MAF and Throttle body, clean them both out pretty well but FORGET to clean inside of the MAF (that area that is not visible but kinda down in the MAF). Well i hook everything back up, have CEL reset and all is well (no light on any longer).

About 5-6 days ago, CEL suddenly returns. Auto store once again shows same code as above, this time Im thinking maybe i didn't clean out the MAF 100% (see above paragraph) and i will re-clean it this coming weekend sometime and try it again. Well on the way home today, the CEL just suddenly turns off and stays off (have not done 2nd cleaning yet). Probably less than 50 miles from when it came on the 2nd time and it going off today on its own.

Any thoughts? Is this indicative of something NOT related to the MAF (O2 sensor or something else?) Should i still remove MAF and clean it again? I mean im glad light went off but i feel like it's only a matter of time (few days) until it comes back on again. No observable performance issues that I notice though I dont tend to pay attention to mpg much.

Basically what would cause the CEL to turn on for 30-40 miles then suddenly turn off that is related to P0171 too lean code?

 

[AntleredRuin]

Too lean pretty much means that there's more air than necessary, usually after the MAF (whose purpose is to tell the engine how much fuel it needs in relation to the air coming in).
So, it could be a vacuum leak, after the MAF. A few weeks ago, I didn't tighten a hose clamp down on a vacuum hose going to my EVAP system after I did my brake booster and I threw a P0171 code in about 10 miles. Tightened it back up, and the code went away.
The system had way more air than the MAF said it did, so it ran lean.


It could be your front O2 sensor, which is what tells the engine that there's an issue in the first place. Maybe the sensor just isn't reading right.


It could also be that the system can't supply enough fuel to meet the air supply needs.
In which case, you'd need to look at injectors, fuel filter, Fuel Pressure Regulator or Fuel Pump. Potentially a clogged fuel line, but unlikely, because you'd feel it stutter.
If it is a fuel issue, it'll eventually get worse and worse and you'll start having driveability/starting issues.
I had a bad FPR that threw the code, and then 3 years later, a bad fuel pump that threw the code again. Both of those cause starting issues, but it drove fine with the failing fuel pump (I was lucky though)


I'd recommend getting an OBDII Bluetooth scanner and the TorquePro app. Around $20 total. That way you don't need to go to the parts store to scan your CEL each time. It will also show pending codes that havent triggered the light yet.

 

[Tundra9er]

Too lean pretty much means that there's more air than necessary, usually after the MAF (whose purpose is to tell the engine how much fuel it needs in relation to the air coming in).
So, it could be a vacuum leak, after the MAF. A few weeks ago, I didn't tighten a hose clamp down on a vacuum hose going to my EVAP system after I did my brake booster and I threw a P0171 code in about 10 miles. Tightened it back up, and the code went away.
The system had way more air than the MAF said it did, so it ran lean.


It could be your front O2 sensor, which is what tells the engine that there's an issue in the first place. Maybe the sensor just isn't reading right.


It could also be that the system can't supply enough fuel to meet the air supply needs.
In which case, you'd need to look at injectors, fuel filter, Fuel Pressure Regulator or Fuel Pump. Potentially a clogged fuel line, but unlikely, because you'd feel it stutter.
If it is a fuel issue, it'll eventually get worse and worse and you'll start having driveability/starting issues.
I had a bad FPR that threw the code, and then 3 years later, a bad fuel pump that threw the code again. Both of those cause starting issues, but it drove fine with the failing fuel pump (I was lucky though)


I'd recommend getting an OBDII Bluetooth scanner and the TorquePro app. Around $20 total. That way you don't need to go to the parts store to scan your CEL each time. It will also show pending codes that havent triggered the light yet.

Okay so basically the fact that it turned on then a few miles later (relatively speaking) it turned off doesn't really indicate much....could easily still be one of the 8-10 things that code will throw on. Thanks for the info, guess ill start trouble shooting and look into the OBDII combo you mentioned.

 

[AntleredRuin]

Nothing really specific, no, but my gut feeling is the O2 sensor. 170k miles is a good long life for an O2 sensor.
Start watching your MPGs also.

 

[gamefreakgc]

Top 3:

1. Vacuum leak (I'd put money on this one)
2. O2 sensor failing
3. MAF gone bad. Not dirty, but failing. Dirty MAFs just kill your gas mileage.

This will help ya with finding a vacuum leak:

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[evaruni]

Im having the same issue with my 2002 4Runner with about 178k miles, I haven’t really noticed any performance changes at all. cleaned the MAF and replaced the o2 sensor and AF sensors w OEM denso parts and then cleared the CEL with my code reader and the light came back after driving around 200 miles, I’ve seen some say that it could be an issue with a loose gas cap and since I haven’t noticed anything wrong with the engine actually running, I’m gonna try that next and see what happens and I’ll probably clean the IAC and TB while I’m at it as well. I’ll post an update if I end up solving, but any input would be greatly appreciated as well

 

[gamefreakgc]

Im having the same issue with my 2002 4Runner with about 178k miles, I haven’t really noticed any performance changes at all. cleaned the MAF and replaced the o2 sensor and AF sensors w OEM denso parts and then cleared the CEL with my code reader and the light came back after driving around 200 miles, I’ve seen some say that it could be an issue with a loose gas cap and since I haven’t noticed anything wrong with the engine actually running, I’m gonna try that next and see what happens and I’ll probably clean the IAC and TB while I’m at it as well. I’ll post an update if I end up solving, but any input would be greatly appreciated as well

Try the vacuum leak detection method with carb cleaner or unlit propane like linked below before you end up spending more money on parts. Usually it's a vacuum leak from a cracked hose or failing gasket on the intake or upper plenum.

 

[dvachon]

Try the vacuum leak detection method with carb cleaner or unlit propane like linked below before you end up spending more money on parts. Usually it's a vacuum leak from a cracked hose or failing gasket on the intake or upper plenum.

thanks

 

[gamefreakgc]

Beg, borrow, steal, or construct, a smoke machine.
You'll save time chasing things that aren't the issue

After all these years I still don't have one. I heard they are irreplaceable for detecting exhaust leaks though.

 

[Bad Luck]

After all these years I still don't have one. I heard they are irreplaceable for detecting exhaust leaks though.

They're great for exhausts leaks! I still use them occasionally for intake leaks, but most of the time a can of throttle body cleaner finds it first for me.

 

[spartacus]

Im having the same issue with my 2002 4Runner with about 178k miles, I haven’t really noticed any performance changes at all. cleaned the MAF and replaced the o2 sensor and AF sensors w OEM denso parts and then cleared the CEL with my code reader and the light came back after driving around 200 miles, I’ve seen some say that it could be an issue with a loose gas cap and since I haven’t noticed anything wrong with the engine actually running, I’m gonna try that next and see what happens and I’ll probably clean the IAC and TB while I’m at it as well. I’ll post an update if I end up solving, but any input would be greatly appreciated as well

Check the vacuum hoses for cracks etc. Also Wouldn't hurt to change the pcv valve and grommet.

 

[jgue467]

It was this hose causing the p0171 on my '01, new oem hose and solved. May need to spray some carb cleaner on folds or just try to bend them back forth to see condition closer. Luck.