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Old 09-29-2021, 11:34 AM #1
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P0171 - System Too Lean (Bank 1) After MAF CLeaning

Hey Guys! Hope your week is going well.

Recently, I cleaned my MAF Sensor and Throttle Body as a preventative measure on my 2002 4Runner with \~148K miles (California). Before I did, I have never had a CEL on. The truck ran well but 3 days after, while I was accelerating on to a freeway that is on an incline, I saw the CEL come on. The truck drove fine even afterwards and I would not have noticed anything was wrong if the CEL did not come on; no crazy rough idling, etc, but maybe a bit weak on the throttle response at high speeds.

I ran the code and got **P0171 - System Too Lean (Bank 1).** I’ve done a lot of research here on the forums and it seems that most of the issues with this code comes from MAF Sensor issues and I did just get the MAF Sensor cleaned so I thought maybe the ECU needed resetting so I disconnected and reconnected the battery. The CEL went away for about a week (~70 miles of driving) and at the exact spot on the freeway entrance, the CEL came on again.



When I took it to the my mechanic at RPM Garage in Monrovia, CA who performed the MAF Sensor & Throttle Body Cleaning, they said that they could not find any clear sign of major vacuum leaks but the LT Fuel Trim reads high around 35\~38 under load, accelerating on a hill. It is their opinion that it is not the MAF Sensor that was recently cleaned but problem with the Fuel Pressure Regulator and the Fuel Filter, which were still factory.


The mechanic who also owns a 3rd Gen 4Runner said that he had the same issue when he cleaned his MAF Sensor, and he said that the computer can start to freak out when the readings are so different after a dirty MAF Sensor has been cleaned. I don’t have an unlimited budget so I am trying to figure out the next sensible move to fix the code? Should I get a new OEM MAF Sensor or try the OEM Fuel Pressure Regulator and Fuel Filter as my mechanic suggested?


Thanks a bunch!
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Old 09-29-2021, 11:41 AM #2
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P0171 - System Too Lean (Bank 1) After MAF CLeaning

Quote:
Originally Posted by Yung_Ji View Post
Hey Guys! Hope your week is going well.

Recently, I cleaned my MAF Sensor and Throttle Body as a preventative measure on my 2002 4Runner with \~148K miles (California). Before I did, I have never had a CEL on. The truck ran well but 3 days after, while I was accelerating on to a freeway that is on an incline, I saw the CEL come on. The truck drove fine even afterwards and I would not have noticed anything was wrong if the CEL did not come on; no crazy rough idling, etc, but maybe a bit weak on the throttle response at high speeds.

I ran the code and got **P0171 - System Too Lean (Bank 1).** I’ve done a lot of research here on the forums and it seems that most of the issues with this code comes from MAF Sensor issues and I did just get the MAF Sensor cleaned so I thought maybe the ECU needed resetting so I disconnected and reconnected the battery. The CEL went away for about a week (~70 miles of driving) and at the exact spot on the freeway entrance, the CEL came on again.



When I took it to the my mechanic at RPM Garage in Monrovia, CA who performed the MAF Sensor & Throttle Body Cleaning, they said that they could not find any clear sign of major vacuum leaks but the LT Fuel Trim reads high around 35\~38 under load, accelerating on a hill. It is their opinion that it is not the MAF Sensor that was recently cleaned but problem with the Fuel Pressure Regulator and the Fuel Filter, which were still factory.


The mechanic who also owns a 3rd Gen 4Runner said that he had the same issue when he cleaned his MAF Sensor, and he said that the computer can start to freak out when the readings are so different after a dirty MAF Sensor has been cleaned. I don’t have an unlimited budget so I am trying to figure out the next sensible move to fix the code? Should I get a new OEM MAF Sensor or try the OEM Fuel Pressure Regulator and Fuel Filter as my mechanic suggested?


Thanks a bunch!
I had the exact same symptom after cleaning throttle body and MAF replacement. I'd get a P0171 during hard acceleration (merging onto freeway).

FWIW, it turns out I had a very small (and hard to detect) vacuum leak. I'd consider spending some time doing a "spray and listen" test before spending money on parts. Some silicone rescue tape solved my issue until I was able to replace the hose.

That said, if your fuel filter is original, it probably is time to replace.

YMMV and best of luck!
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Old 09-29-2021, 12:31 PM #3
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It developed an air intake leak during the maf cleanout process, bummer.

My intake leaked from the large 'hose' just before the metal intake runners start. New OEM part and done. So test the different sections of hose between maf and metal runners, but I'd suspect the one that takes the most abuse and heat, the closest one. Luck.
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Old 09-29-2021, 04:42 PM #4
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Quote:
Originally Posted by cl4Rk View Post
I had the exact same symptom after cleaning throttle body and MAF replacement. I'd get a P0171 during hard acceleration (merging onto freeway).

FWIW, it turns out I had a very small (and hard to detect) vacuum leak. I'd consider spending some time doing a "spray and listen" test before spending money on parts. Some silicone rescue tape solved my issue until I was able to replace the hose.

That said, if your fuel filter is original, it probably is time to replace.

YMMV and best of luck!
Thanks for the reply! Seems like you did indeed have a very similar issue. Do you happen to remember the place where the vacuum leak was? Or the part number for the hose you replaced?
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Old 09-29-2021, 04:47 PM #5
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Quote:
Originally Posted by Yung_Ji View Post
Thanks for the reply! Seems like you did indeed have a very similar issue. Do you happen to remember the place where the vacuum leak was? Or the part number for the hose you replaced?
Well, definitely check everything past the airbox, like @jgue467 said, very well could be that elbow immediately before the throttle body.

IIRC, my leak was one of the smaller hoses on top of the intake. I didn't need a Toyota part, just a length of rubber hose from the auto parts store which I cut to fit.
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Old 09-29-2021, 05:29 PM #6
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It's a vacuum leak. If you can, double check the hoses on the throttle body are secure and have no visible cracks. Also make sure the MAF sensor itself is seated securely with no leaks. Here's a great how-to on how to check for leaks, it only costs $3 for a can of carb cleaner:

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Old 09-29-2021, 06:57 PM #7
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I had to replace the intake tract hoses on mine from decay. Just change the mass airflow sensor, they’re not that expensive. They take a lot of abuse over the years and survive impressively well for how delicate of a part they actually are.
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Old 09-29-2021, 07:32 PM #8
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Quote:
Originally Posted by gamefreakgc View Post
It's a vacuum leak. If you can, double check the hoses on the throttle body are secure and have no visible cracks. Also make sure the MAF sensor itself is seated securely with no leaks. Here's a great how-to on how to check for leaks, it only costs $3 for a can of carb cleaner:

Thanks for your input, it seems like during the MAF Sensor cleaning & Throttle Body Cleaning, residue got in the MAF Sensor or a vacuum leak happened somewhere. Before going Fuel Filter + Fuel Pressure Regulator, I will get a new OEM MAF Sensor and check thoroughly for vacuum leaks.

I'm also hearing some people say even a torn boot will throw that CEL code!!
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Old 09-29-2021, 08:38 PM #9
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Quote:
Originally Posted by cl4Rk View Post
Well, definitely check everything past the airbox, like @jgue467 said, very well could be that elbow immediately before the throttle body.

IIRC, my leak was one of the smaller hoses on top of the intake. I didn't need a Toyota part, just a length of rubber hose from the auto parts store which I cut to fit.
Just ordered Denso MAF Sensor, and I will take a look for leaks in that area! Did you get it smoke tested or just used a carbon cleaner spray??
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Old 09-29-2021, 08:48 PM #10
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P0171 - System Too Lean (Bank 1) After MAF CLeaning

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Originally Posted by Yung_Ji View Post
Just ordered Denso MAF Sensor, and I will take a look for leaks in that area! Did you get it smoke tested or just used a carbon cleaner spray??

Carb cleaner, listened for things to get temporarily bogged down. Also kept one eye on my OBD2 phone app to confirm RPM change. Wait for the idle to smooth out after start, take your time, systematically move your way through the hoses. Best of luck
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Old 09-30-2021, 02:35 AM #11
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After running a dirty maf sensor your computer learns to adjust the air fuel ratio to unwanted values. I just hit 100 thousand miles. Cleaned maf, throttle body, idle control. 4runner needs to be relearned! The is a procedure that involves using the key in the ignition in a certain sequence. The fuse and battery methods don't work. After relearned my 4runner idled correctly, ran better and stronger, and gas mileage increased.
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Old 09-30-2021, 10:00 AM #12
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Originally Posted by Jakeepoo3 View Post
After running a dirty maf sensor your computer learns to adjust the air fuel ratio to unwanted values. I just hit 100 thousand miles. Cleaned maf, throttle body, idle control. 4runner needs to be relearned! The is a procedure that involves using the key in the ignition in a certain sequence. The fuse and battery methods don't work. After relearned my 4runner idled correctly, ran better and stronger, and gas mileage increased.
Disconnecting the battery overnight worked for me. What's the key sequence you used?

Edit: It looks like you own a 2010 4Runner so your 5th Gen 4Runner shares very little in common with all the 3rd gen's in this subforum.
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Old 09-30-2021, 03:17 PM #13
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Quote:
Originally Posted by Bad Luck View Post
Disconnecting the battery overnight worked for me. What's the key sequence you used?

Edit: It looks like you own a 2010 4Runner so your 5th Gen 4Runner shares very little in common with all the 3rd gen's in this subforum.

Did it not work when you disconnected the battery for only 15 minutes? Only when you disconnected it overnight??
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Old 09-30-2021, 03:31 PM #14
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Did it not work when you disconnected the battery for only 15 minutes? Only when you disconnected it overnight??
I had the option to leave it disconnected overnight so that's what I did. I didn't attempt any other time period
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Old 09-30-2021, 03:35 PM #15
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Quote:
Originally Posted by Jakeepoo3 View Post
After running a dirty maf sensor your computer learns to adjust the air fuel ratio to unwanted values. I just hit 100 thousand miles. Cleaned maf, throttle body, idle control. 4runner needs to be relearned! The is a procedure that involves using the key in the ignition in a certain sequence. The fuse and battery methods don't work. After relearned my 4runner idled correctly, ran better and stronger, and gas mileage increased.
Interesting, I never heard about this key sequence thing on the 3rd Gens… Anyone else know anything about this?
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