Need Help - Code P0171 Poor MPG

Wheelie

Wheelie

New member
Hi to all. First post. Just need to make a quick check with someone more knowledgeable about this before I jump in to start replacing more stuff.

1999 V6 4Runner with 149,000 miles. Bought 3 months ago for 16 yo son.

  • Runs fine hot or cold
  • Tail pipe color normal
  • Gas mileage is poor (about 14 MPG) I understand it should be 17 to 19 MPG
  • Month ago it threw the check engine light (Code P0171 System Too Lean Bank 1) and so I replaced the air/fuel sensor (oxygen sensor)
  • Two weeks later the check engine light came on again. ODB code reader shows same code (P0171)
  • I removed and cleaned the mass flow sensor and two weeks later same code came back
  • Air filter newish and very clean
  • New spark plugs 25,000 miles ago
  • Passed emissions 2,000 miles ago
  • TPS (throttle pos sensor) replaced a year ago
  • No abnormal oil consumption

Anyone experienced this or maybe has better insight into this problem? Maybe a fuel injector? Fuel filter?
 
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Im no pro but sounds like a clogged catalytic converter to me. System to lean codes seem to just revolve around eachother. Bad 02 sensors, bad cats.

Which 02 did you replace? Upstream or downstream?
 
Thanks for the response!

Upstream sensor replaced.

Hmmm. Could be the CC. But the motor runs fine. Isn't a clogged CC usually associated with lack of power, bad smell, high engine temps, rough idle, and stumbling acceleration? Plus it just passed emissions test. I do have poor fuel economy, but it runs absolutely fine.

Guess I could pressure test it like this pretty easy: How to check your vehicle's catalytic converter. - YouTube
 
Baja Runner said:
Its the MAF... Replace it and you're done.
Click to expand...

How do you feel so confident that that's what it is..? I agree that it could be, but before he goes out and buys a new one I think you should offer him some guidance on how to test it, or perhaps some more information on why you're so certain that this is the cause.

I also think it's the MAF, however, I am not sure how you could test it...
 
A.Wilson013 said:
How do you feel so confident that that's what it is..? I agree that it could be, but before he goes out and buys a new one I think you should offer him some guidance on how to test it, or perhaps some more information on why you're so certain that this is the cause.

I also think it's the MAF, however, I am not sure how you could test it...
Click to expand...

Usually I don't just say stuff like that, but after having several 4runners with the 3.4L over 12 years, and being on Toyota forums, I feel pretty confident.

Also the fact that the CEL was off after he cleaned it, might point to a MAF. I know you can test the MAF with a multimeter.
 
Thanks for the tips. The MAF certainly sounds plausible.

I will pick up a new MAF and Fuel Filter today and try to install both tonight and let you know if the CEL comes back on again.
 
Please post back with what happens because its good to have this stuff documented.

A lot of people forget to follow up with their fixes!
 
The only true reliable way to test it is to use a Toyota scantool and watch calculated load while doing a stall test. If load doesn't reach 100% it's definitely a bad MAF. I'm just not sure of access to a scantool without the ~$100 diag fee.
 
Baja Runner said:
Please post back with what happens because its good to have this stuff documented.

A lot of people forget to follow up with their fixes!
Click to expand...

I will report back. I installed the MAF a couple hours ago and it drives noticeably better (more responsive). Topped off the fuel tank and recorded the mileage. Will report back later this week at the next tank refill to see if she gets back to 18 MPG or so. Hopefully the CEL will stay off as well.

Thanks!
 
Wheelie,
When you post back mileage would you also say the part number for the MAF and possibly a small write up on replacing it? I have the PO171 issue, and have only tried cleaning it. Would appreciate the help!
 
ameining said:
Wheelie,
When you post back mileage would you also say the part number for the MAF and possibly a small write up on replacing it? I have the PO171 issue, and have only tried cleaning it. Would appreciate the help!
Click to expand...

I just removed the cruise control module (3 - 10mm bolts)and pushed it out of the way towards the firewall and then removed the MAF from the plenum (held in place with 2 phillips head screws).

Advance Auto refurb Part No. 74-50009 $80

This guy removes a lot more stuff than I had to : P0171 P0174 Toyota 4Runner MAF Mass Air Flow Sensor Fix - YouTube
 
ameining said:
Wheelie,
When you post back mileage would you also say the part number for the MAF and possibly a small write up on replacing it? I have the PO171 issue, and have only tried cleaning it. Would appreciate the help!
Click to expand...

If I helps the part number should be emboss right on the top of the MAF and there are only 2 small screws holding it in. Be careful with the screws, they like to strip.
 
Baja Runner said:
Please post back with what happens because its good to have this stuff documented.

A lot of people forget to follow up with their fixes!
Click to expand...

As requested:
  • Oxygen sensor replaced - did not work (code came back in about 2 weeks)
  • MAF cleaning - did not work (code came back in about 2 weeks)
  • MAF replacement improved engine starting and performance immediately
  • 1st tank of fuel no MPG change (about 14 MPG)
  • Average MPG for next two tanks of fuel = 20 MPG!!

:party: Success!! :party:
 
Wheelie said:
As requested:
  • Oxygen sensor replaced - did not work (code came back in about 2 weeks)
  • MAF cleaning - did not work (code came back in about 2 weeks)
  • MAF replacement improved engine starting and performance immediately
  • 1st tank of fuel no MPG change (about 14 MPG)
  • Average MPG for next two tanks of fuel = 20 MPG!!

:party: Success!! :party:
Click to expand...

Been reading for a while and doing troubleshooting on my T4R that seems to have a head or head gasket issue (lots of white smoke but no coolant consumption). Getting P0172 code - Bank 2 running rich on fuel, pretty bad MPG and gas smell at the exhaust. At some point cleaned my MAF and noticed some improvement but did not replace it as it was "ok" as per test done following Hane's manual. Will get it scanned hopefully tomorrow... let's see what's next. :confused:
 
For my 2002 4Runner Limited 4WD the MAF P/N embossed on the sensor is Denso 22204-21010 197400-2000. A google search says these are available in China. Thru the Denso web site they say Denso P/N 197-6020 applies. Amazon has this P/N available but states that it's not applicable for my 4Runner.....
Thoughts?
 
CitadelBlue said:
For my 2002 4Runner Limited 4WD the MAF P/N embossed on the sensor is Denso 22204-21010 197400-2000. A google search says these are available in China. Thru the Denso web site they say Denso P/N 197-6020 applies. Amazon has this P/N available but states that it's not applicable for my 4Runner.....
Thoughts?
Click to expand...

After cleaning the MAF with the store bought spray and then a second time with alcohol ... I purchased a new Denso 197-6020 Mass Air Flow Sensor
and installed a new air filter .. I cleared the CEL and after 50 miles of driving no CEL ..... hope this was the fix .....
 
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