P0171 after Throttle Body and IAC valve cleaning

[Dezert4Runner]

My idle was around 500 rpms and I am due for CA smog, so over the weekend I cleaned the Throttle body and IAC valve to get it running as well as possible for the smog test. They were filthy, and they were removed from the vehicle and cleaned thouroughly before going back on the motor. Now the idle is around 900-1k rpms, which is a little high. Overall the truck has never ran better though. The only problem is that after a couple miles of driving it, I now have a CEL (P0171) and had previously never had a CEL during my 1.5 years of ownership.


My specs: 2000 5vz-fe 5 speed 4wd (cali spec) with 325k+ (gauge cluster replaced at some point)


Here's a list of things recently done:


In the last week:

3/11/17 -Cleaned throttle body and IAC with CRC Throttle Body Cleaner & replaced IAC gasket with Felpro gasket. Also cleaned MAF sensor with CRC MAF Sensor cleaner twice. Yes, I did get the wires down inside of the sensor with the straw attached! I had pulled the EFI fuse for a couple hours while doing this work. CEL came on after a short drive, I cleared it with my scan tool, and within a coupe miles it came back.

3/13/17 -Replaced 1 of the vacuum hoses going from the IAC to the air intake box that had a couple cracks in it. Checked all other vacuum hoses for cracks and that they were all connected and tight. CEL came back on within a few miles.

3/14/17 -Brand new paper Purolator air filter. CEL came back in within 20 miles.


In the last 3k miles:

Done 2-3k ago -New dual electrode NGK plugs and NGK TE66 plug wire set. New Toyota PCV valve and grommet.

Done 1k ago -Oil and oil filter changed last month, in case it's relevant.


MAF is operating ok per this thread http://www.toyota-4runner.org/3rd-gen-t4rs/141851-p0171-lean-bank-1-question.html

Based off of lots of research on T4R.org, the next probable culprit is the upstream A/F Sensor. I don't have a multimeter to check resistance. Should I just go ahead and replace it? I understand that a P0171 could be air or fuel related and my OBD2 reader (ELM327 wifi scanner) doesn't monitor o2 or fuel pressure, so not sure if I should just start with the upstream A/F sensor since it's due anyway and go from there. I have not done the fuel filter while in my possession, so that's also something I'm thinking about replacing. The cheapest I saw the Denso 234-9001 A/F Sensor is about $80 from Amazon and Wal Mart and I'd rather not throw parts at it unless necessary.

Thoughts?

 

[Dezert4Runner]

Some before & after photos for anyone interested in the TB & IAC cleaning process.

Intake Plenum

Metal TB gasket

I had cleaned the front side of the TB about a year ago while on the truck so it wasn't too bad

The back side, different story

Underside of the TB where the IAC mounts up

IAC

Felpro IAC gasket top, OEM on bottom

Metal TB gasket and intake plenum cleaned up

Bottom of TB cleaned up

IAC cleaned up

 

[ZipZapZot]

Had a similar issue recently. Cleaned the replaced MAF sensor but code persisted. Replaced IAC with a generic version. Still had the P0171 error issue until I replaced it with an OEM Denso unit and that finally resolved the problem for me

Sent from my SM-G920V using Tapatalk

 

[baja1]

well thats great that the IAC is clean as a whistle... while it was off did you verify it was actually working by applying voltage to the coil?

 

[Dezert4Runner]

well thats great that the IAC is clean as a whistle... while it was off did you verify it was actually working by applying voltage to the coil?

I read a "how to clean the IAC" thread on here about applying voltage to test it but that person also saw a spark and the IAC was toast, so I didn't want to risk accidentally frying it. The idle is smoother and the cleaning seemed to help a lot, so I'm going off of that and calling it good. There was no code being thrown before I cleaned it, so that would imply that it was ruined by cleaning it. I guess that's a possibility.

 

[Dezert4Runner]

On another note, I checked my MAF sensor readings again this afternoon on a couple of different occasions, and they were lower than what is considered acceptable. Here's a quote from the thread I linked in the OP, along with some readings I took from the Engine Link App on my way home from work. Note that this was following the procedure in the link regarding no load during these readings.

Below is a straight copy/paste of diagnostic procedures in fairly basic terms with limited tools. From Identifix/IATN. The numbers next to different items at the bottom show the number of technicians reporting to have fixed this code with each of the following parts. The vast majority (including my own vehicle) of these I've seen are simply due to a dirty MAF sensor and can be easily resolved with cleaning. I have seen many many people clean them incorrectly thinking mistaking the intake air temp posts for the MAF heater wires. What brand MAF sensor did you replace with... wouldn't be the first time I've seen the code return due to an inferior part. Also on you emissions tag under the hood is it listed as Federal or California Emissions? If it is a California vehicle its a bit of a different approach. Let me know if you need anything or have any further questions and we'll try to get it worked out.


1999 Toyota 4Runner 3.4L, Eng Cfg V6, Eng Des 5VZFE, USA/Canada

Hotline Archive
With OEM Direct diagrams, components, TSBs, and R&R procedures

Number: 482996
Vehicle Application:
1998 4Runner 3.4 1999 Tacoma 3.4
1999 4Runner 3.4 2000 Tacoma 3.4
2000 4Runner 3.4 2000 Tundra 3.4

Customer Concern:
Has a check engine light on and a code P0171.

Tests/Procedures:
1. Monitor the fuel trim readings at idle and at 55 MPH driving down the road. If the numbers are low at idle but climb on a part throttle cruise, the airflow meter could be reading low or it may also have low fuel pressure. The mass air flow sensor grams per second should be 3.3-4.7 at idle and 12.9-18.3 at 2500 RPMs no load, if the readings are low the sensor could be bad.

2. Monitor the calculated load reading on a wide-open throttle acceleration. The reading should go to 85% or more. If not, then the airflow meter could be faulty.

3. Monitor the oxygen sensor inputs, both front and rear oxygen sensor signals. The oxygen sensor should not stay high or low voltage for extended periods of time. If the oxygen sensor shows low voltage and the fuel trim readings climb, check the performance of the oxygen sensor.

4. Check the vehicle for a vacuum leak by spraying carburetor spray around the intake manifold while watching the fuel trim readings. If the readings change, there is a vacuum leak.
Tech Tips:
If fuel trim readings are high at idle and go down on a free rev, there is a vacuum leak. If the fuel trim readings are high at idle and at cruise, suspect fuel injectors.


Diagnostic Codes
P0171

Confirmed Fix
225 - Mass Air Flow (MAF) Sensor
9 - Engine Air Filter, Mass Air Flow (MAF) Sensor
4 - Oxygen (O2) Sensor(s)
2 - Fuel Pump
2 - Heated Oxygen Sensor (HO2S) Bank 1 Sensor 2 (B1S2)
2 - Heated Oxygen Sensor (HO2S) Bank 1 Sensor 1 (B1S1)
1 - Front Oxygen (O2) Sensor
1 - Vacuum Leak
1 - Fuel Injector(s), Mass Air Flow (MAF) Sensor
1 - Cruise Control Vacuum Line
1 - Intake Manifold, Mass Air Flow (MAF) Sensor
1 - Mass Air Flow (MAF) Sensor, Throttle Body Cleaning Procedure
1 - Mass Air Flow (MAF) Sensor, Mass Air Flow (MAF) Sensor O-Ring
1 - Exhaust Pipe, Mass Air Flow (MAF) Sensor
1 - Air Intake Tube
1 - Mass Air Flow (MAF) Sensor, Throttle Body
1 - Air Fuel Ratio (AFR) Sensor
1 - Engine Vacuum
1 - Oxygen (O2) Sensor, Bank 1 Sensor 2

My reading @ idle: 3.21

My reading at 2,500 rpm: 10.04


I found that these above readings were similar after trying this a coupe times this evening, so now the MAF sensor seems to have a low output, indicating that it may need replacement.

 

[Dezert4Runner]

Just installed a new Denso MAF sensor that I ordered off of Amazon. The readings are nearly identical to the original Denso that I took out! I cleared the CEL via my Engine Link app, and the light came back on within a couple miles. Something smells fishy about the new one not reading higher than it is. Here's a couple frame grabs from the Engine Link App.

At idle (orig MAF)

At idle (new MAF)

At 2500 rpm (orig MAF)

At 2500 rpm (new MAF)

 

[LittleCaesar]

If you app doesn't read Live Data for the Oxygen or A/F sensor, find a scan tool that does. It has probably gotten lazy over time. If has never been replaced I'd start there though like yourself I hate firing the parts canon.

 

[AZBub]

Tried checking for intake leaks with carb cleaner or exhaust leaks with the shop vac method? If not your MAF its time to think leak somwhere.


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[Dezert4Runner]

Forgot to update this post. After throwing parts at it (MAF and A/F sensors), the A/F sensor ended up doing the trick. CEL hasn't returned after another 1k+ miles. Apparently, CA doesn't do a "sniffer" test on 2000 and newer vehicles, so (I believe) all they do is check the system for readiness monitors and the usual visual inspection under the hood. Actually just got my new registration & sticker in the mail yesterday :reddance:

 

[k2damfc]

I’m having same problems.
Been having an idle problem (after idling for a few mins would just stall over and over) so decided to clean the TB and once reading into decided the IAC too. Didn’t test IAC however, but snowed right after I did it so it will have to wait.
Now the idle is erratic, not always high but drops low and sounds like miss firing a bit. All new symptoms since the cleaning. When reved up she sounded great and drove great. But stopped at a light it idles just rough. Then last night stalled a bit while driving around. Getting p0171 codes now. What can be done to test the AF sensor to see if that’s a problem? New IAC from Napa is $225 and haven’t even asked the dealer. Considering I just read that the oem one fixed issues I’m leaning more to getting that instead but $$$. Going to apply voltage to it as soon as I can rip it out again. Just curious on other steps to take.
Great to clean something to help it run better and now it’s all messed up lol.

 

[gamefreakgc]

I’m having same problems.
Been having an idle problem (after idling for a few mins would just stall over and over) so decided to clean the TB and once reading into decided the IAC too. Didn’t test IAC however, but snowed right after I did it so it will have to wait.
Now the idle is erratic, not always high but drops low and sounds like miss firing a bit. All new symptoms since the cleaning. When reved up she sounded great and drove great. But stopped at a light it idles just rough. Then last night stalled a bit while driving around. Getting p0171 codes now. What can be done to test the AF sensor to see if that’s a problem? New IAC from Napa is $225 and haven’t even asked the dealer. Considering I just read that the oem one fixed issues I’m leaning more to getting that instead but $$$. Going to apply voltage to it as soon as I can rip it out again. Just curious on other steps to take.
Great to clean something to help it run better and now it’s all messed up lol.

What also can happen is during the cleaning of the IAC, sediment falls into the valve and makes it stick so it doesn't open/close as it should.

The P0171 code normally is a vacuum leak, especially after working on the vehicle. Double check all hoses are secure and correctly routed as well.

 

[k2damfc]

So a rushed jobbed is a f**ked job. I was doin all the work outside while it was warm enough to do so however it meant high winds at the time so I rushed.

Bottom line: I put TB gasket on wrong and was causing my issues. #facepalm

Was hoping the cleaning would fix a problem I had before but did not. So while using my transmission to slow me down on the steep downhill sections in Colorado, the rpms start climbing as you would expect it too. But when I get around 3k-3.5k the rpms drop a little and pressing the gas pedal gives no response unless I give it serious gas, which is not ideal on snowy roads, and picks up and revs high and jerks us around. Only happens in low and 2, automatic, kind of annoying more then a major issue. Just wondering someone’s 2 cents on that. TPS? Regulator hooked up to TB? Just dunno where to even start, after cleaning TB, IAC, MAF and new filter. Still gotta get pcv replaced too.

 

[gamefreakgc]

So a rushed jobbed is a f**ked job. I was doin all the work outside while it was warm enough to do so however it meant high winds at the time so I rushed.

Bottom line: I put TB gasket on wrong and was causing my issues. #facepalm

Was hoping the cleaning would fix a problem I had before but did not. So while using my transmission to slow me down on the steep downhill sections in Colorado, the rpms start climbing as you would expect it too. But when I get around 3k-3.5k the rpms drop a little and pressing the gas pedal gives no response unless I give it serious gas, which is not ideal on snowy roads, and picks up and revs high and jerks us around. Only happens in low and 2, automatic, kind of annoying more then a major issue. Just wondering someone’s 2 cents on that. TPS? Regulator hooked up to TB? Just dunno where to even start, after cleaning TB, IAC, MAF and new filter. Still gotta get pcv replaced too.

I can't say I've ever downshifted to 2 or Low on the transmission shifter to slow down on hills. I used to do that in my manual, but then again it's a manual and all you have to do it let off the gas. An auto always has some throttle applied to keep the engine from stalling.

As long as it doesn't do that when shifted in D, I think what's happening is the throttle has to be open enough to overcome the 3k-3.5k the engine is already spinning due to the transmission being in gear and the torque converter locked. The ECU probably has to see enough throttle to push the RPM's over 3.5k which is why it could be screaming to life. Not really sure an auto transmission is meant to be used like that. I would shift back to D before giving it gas again.