sjl_jim

Hello everyone, this forum seems to be a great source of information. I stumbled upon it thru a Google search in the quest to troubleshoot my dad's 00 4Runner P0171 issue.

I just wanted to give a little history on the condition under which the code was set and get your diagnosis.
About a month ago, my dad got an oil change done at Wal-Mart supercenter as he had done many times before. This last time, he picked up his truck and couple days later, on I-25, he got the "check engine" light. He got off the next exit at a gas station to check under the hood. Now, my dad is not the mechanic type but he noticed that he was missing his engine oil cap Fortunately, there wasn't a whole lot of oil spilled and there was nothing he could do there, so he drove home. Then, he visited Wal-Mart and had I'm sure an unpleasant discussion with the car care manager. Anyway to make a long story short, they replaced the cap but now he is plagued with this P0171 code that won't go away.
He told me that he had it cleared by a mechanic (recommended to him by a friend) but it keeps coming back. He also noticed that he is getting worse gas mileage than before and that at hard acceleration, the truck seems to be bogging.

From the search I've done on this forum, it sounds like it's either "MAF needs to be cleaned" or "the pre-cat O2 sensor (aka A/F ration sensor?) is faulty". My dad is willing to take it to the recommended mechanic but he would like to pin point the actual part that needs to be replaced. He just didn't want to get in the mode of replacing all the emission components until the problem goes away. I wanted to take a closer look before I have him break down and take it to the shop. I wanted to get some feedback from this forum before digging into it this week.
I would appreciate any input you may have on whether I am on the right track or not... Also, a recommendation on an online store for Toyota parts would be great.

Thanks in advance,

Jim

midiwall

A breakout of the codes is here:
https://www.yotatech.com/~corey/tech/dr_z_obd/dr_z.htm

From there, a P0171 is "system too lean, bank 1". "Bank 1" defines that this code is coming from the O2 sensor that's in front of the catalytic converter. (a similar code, P0174, would point to the O2 sensor after the cat) And yes, the O2 sensor is what the engine computer ("ECU") uses to compute the Air/Fuel Ratio.

There are a slew of codes (P0130 - P0167) that point to issues with the sensor itself, so that's not the place to start. I would guess that the sensor is "fine", though it may not be 100% perfect.


Anyway, what you've been told thus far is good info... The place to start here is with the MAF. The cleaning process is easy, and well documented here:
http://www.4x4wire.com/toyota/mainte...tenancep5.html

Past that, the air filter should also be replaced (or cleaned if it's a foam unit).

Try those two things first before replacing the O2 sensor, it's a fairly expensive part ($90) that will turn into a $160 visit to the dealer (parts plus labor).


Oh, you also asked for a favorite online parts dealer...

I've had great luck with these guys:
https://www.1sttoyotaparts.com/partscat.html

But, if you're thinking O2 sensor, then the best prices around are coming from a performance specialist ("Gadget") at URD USA:
http://www.urdusa.com/

sjl_jim

Thanks for the input Mark! I will look thru the links you provided and start with the MAF cleaning and air filter. Forgot to mention that he has a K&N drop in filter that may be getting the sensor oily. I recall reading somewhere that the MAF goes thru a self cleaning cycle everytime the engine is started. Is that process not effective after years of cycles? Like anything else, I'm sure the MAF sensor has it's life expectancy...

Once again, Thanks.
I will update this post up to date of my process....

Jim

rimpainter.com

Wow, how refreshing when a noob does his research!

I am still having trouble getting over this whole oil cap mishap at Walmart. Surely it could be related in some manner? Did oil spray all over under the hood, or did the baffle plate prevent that from happening?

I guess it could be coincidence...

midiwall

Tis can definitely be part of the issue. Any of the oiled filters are susceptible to being over-oiled (it's not a K&N issue) and the excess has only one place to go to - into the intake, which means right past the MAF.

Ummm, that's news to me - there's nothing in the MAF that could "clean" it - it's not like the elements in there heat up and burn off anything foreign. You'll see what I'm talking about once you work through the cleaning.


Well, yes and no. The MAF is sensitive and can get damaged, but in general, it's a long timeframe, for example, mine's original ('96, 137k miles).

Replacing the MAF is a last ditch effort. Even if you find one in a junk yard, you're still looking at $200ish.


Good luck, and yes, please keep us in the loop!

sjl_jim

From what my dad said he was able to clean up the mess with paper towels at the gas station. And I was able to look at the engine bay last time I saw him and it didn't look bad... The baffle must have definitely kept oil from shooting out of the hole.

I was assuming that the cap being off was directly related to the code being thrown. Although, I would have guessed codes related to low oil pressure or something along that line be thrown...

Now, I'm anxious to get my hand on my dad truck to figure out to root cause...

Thanks for the input.

sjl_jim

Mark,
I stand corrected regarding the cleaning cycle. I was remembering an article I had read on www.4x4wires.com. I tried to find the article I had read but no luck. But, I had saved it to a word doc so here is a little quote I was remembering:

"...This process of the wire becoming coated with contaminates is known to the industry and many hot wire MAF sensors now incorporate a "burn off" cycle activated every time the ignition switch is cut off. Sufficient current is passed through the hot wire to briefly raise it to 1000 degrees Centigrade to burn off any contaminants. I have not seen any information in Toyota documentation indicating that this feature is incorporated on the 3rd generation engines under discussion, so this might be the reason this problem occurs. Newer technology MAF sensors use a heated "thin film" versus "hot wire" that is reported to be more resistant to contamination problems."

The statement in blue was what I had NOT recall... :pat: Oops... So, clean, the MAF will be...

Thanks...

midiwall

I dropped a piece of the quote into Google and the 4x4wire article popped up:
http://www.4x4wire.com/toyota/4Runner/tech/OBDII_ECU/

1000 degrees _centigrade_? WOW... yeah, I would imagine that would toast a lot of gunk off the wire!


Good luck with the cleaning, I hope it helps!

sjl_jim

Sorry folks for being out for so long. Just wanted to give an update on the issue my dad was having with the P0171 and check engine light.

So, about 3 weeks ago, I went down to his house and cleaned his MAF per instructions found on this forum. Couldn't get the engine light to go off, so un-plugged the battery for 1 min. and plugged it back in.
Then went for a test drive and the 4runner ran like a champ. Had my dad drive it and the hesitation under WOT he had mentioned before seems to be gone. So, left it at that. Time would tell whether the CEL will come back.

Well, it's been 3 weeks and NO CEL and the runner is running just fine.
Thanks a lot for your help and this forum ROCKS!!!

PirateFins

Glad to hear that it worked out for you and your dad's Runner. Now be a good boy and go buy a Runner or Truck yourself and stick around.

sjl_jim

I already have a truck and probably won't be getting another vehicle for few years. I have a 02 Chevy 2500HD Duramax/Allison CC, SB 4X4 to pull a 25' 5th wheel camper. So, can't go w/ a 4Runner or even Tundra. Although, I have to say that I have driven my dad's Runner on ice fishing trips and I was totally impressed w/ the gas mileage (~25 mpg)!!!! Before my current truck, I had a 00 Durango and whew... was it thirsty!!! (~12 mpg avg). Glad I got rid of that!!!

Thanks for the reply. I'll keep checking in time to time. My dad is going to have his 4Runner for a while...