P0171 - System Too Lean (Bank 1) After MAF CLeaning

[Yung_Ji]

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!

 

[cl4Rk]

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!

 

[jgue467]

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.

 

[Yung_Ji]

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?

 

[cl4Rk]

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 [MENTION=75316]jgue467[/MENTION] 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.

 

[gamefreakgc]

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:

<iframe width="560" height="315" src="https://www.youtube.com/embed/9CPqbaSgcok" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

 

[Romeo1]

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.

 

[Yung_Ji]

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:

<iframe width="560" height="315" src="https://www.youtube.com/embed/9CPqbaSgcok" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

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!!

 

[Yung_Ji]

Well, definitely check everything past the airbox, like [MENTION=75316]jgue467[/MENTION] 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??

 

[cl4Rk]

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

 

[Jakeepoo3]

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.

 

[Bad Luck]

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.

 

[Yung_Ji]

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??

 

[Bad Luck]

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

 

[Yung_Ji]

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?

 

[gamefreakgc]

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??

Just listen to the engine. Do it upon a cold start so if the carb cleaner hits the exhaust pipes it won't combust. Start it up and open the engine bay. Just listen for the engine tone to smooth out and fan to slow down. Then just spray on every vacuum hose end as well as the cracks in the the intake 'accordion' pipe. You'll know if you get it, the engine will either rev up or down quickly as soon as the carb cleaner hits the leak and then return to normal. Then replace the offending part. Any vacuum hose can be matched by an auto parts store but the intake may not be carried by them.

 

[Bad Luck]

Interesting, I never heard about this key sequence thing on the 3rd Gens… Anyone else know anything about this?

Jakeepoo3 has a 2010 4Runner which is a 5th gen. There may be a key sequence for him, but that would not apply to 3rd gen vehicles.

 

[Jakeepoo3]

Jakeepoo3 has a 2010 4Runner which is a 5th gen. There may be a key sequence for him, but that would not apply to 3rd gen vehicles.

You're right about the generation difference. I didn't clarify. It took me a whole day browsing through the internet to find it after trying all the other methods. There's a video on YouTube that describes the procedure. I can't remember how I found it. The guy was in a Toyota car. He said that it would work on most Toyota vehicles. I didn't think it would work but I had already tried all the other stuff so I did it. It worked! Remember when you do get the relearning process started to let the truck idle for like ten minutes with all your accessories off. Then go through every gear in 2wd, 4wd, and 4lo and let idle for a few minutes. I drive mine like a maniac and give her hell for a hundred miles or so to learn my style. Crazy computers. Good luck!

 

[Jakeepoo3]

Actually I found it. Here's the link. Hope this helps you get running properly!

How to reset idle on ANY Toyota car! - YouTube

 

[gamefreakgc]

Interesting, I never heard about this key sequence thing on the 3rd Gens… Anyone else know anything about this?

It is for a 5th Gen. There's very few key sequences for our 3rd Gens and only for late year production vehicles. Too much thinking power for our archaic ECUs