P0171 code has me and my shop stumped so far (its not the MAF)

[ECT_Gumby]

I have read many, many of the threads on here related to 3rd gen 4runners and P0171. New MAF and O2, cleaned intake and fuel system, no vacuum leaks, etc etc.

The truck run fine, 20 mpg, idles at 600 and all that. The shop has logged the data while driving it and what seems to be happening is a blip of bad data comes through and trips the CEL. There is no misfire or anything out of the ordinary, it just comes on.

They seem to think its electrical between a sensor and the computer or the computer its self. IMO if its the computer I would see more tell tail signs or poor performance though. :shrug:

Has anyone else encountered anything like this?

 

[ILRunner98]

Have any specs on this "blip of bad data" by chance? Im interested. Like what data was abnormal?

 

[LittleCaesar]

Both new parts OEM, or at least Denso?

Maybe a ground, or bad ECU. I think Brian_SD has something similar to this last year. Whether he got it figured out I don't know.

 

[ECT_Gumby]

Have any specs on this "blip of bad data" by chance? Im interested. Like what data was abnormal?

Good question, I'll see if I can get some numbers or print outs Tuesday. All they said is a reading from one of the sensors seems to go out of spec for just a second which is long enough to trip the CEL. One time it was just sitting in the parking lot and threw the code. :suspicious:

 

[ILRunner98]

Ok. sometimes thats all it takes to set the check engine light. We had a tacoma at my work with a similar problem where the front O2 sensor would drop its reading for 3 seconds and this caused no throttle response whatsoever. Except in this case it took a long time to find becuase it wouldnt set a code. Im guessing in your case that maybe the coolant temp sensor. Those are known for randomly dropping the signal and causing all sorts of issues.

 

[shootmymime]

mine throws a code rarely (but happened today) and i just reset it sitting in the parking lot running.


mine started after i put the super charger on. do you have one? just a thought?

 

[ECT_Gumby]

Both new parts OEM, or at least Denso?

OEM, I read enough of these forums to know better lol

mine started after i put the super charger on. do you have one? just a thought?

Nah, its all stock

 

[ECT_Gumby]

Ok. sometimes thats all it takes to set the check engine light. We had a tacoma at my work with a similar problem where the front O2 sensor would drop its reading for 3 seconds and this caused no throttle response whatsoever. Except in this case it took a long time to find becuase it wouldnt set a code. Im guessing in your case that maybe the coolant temp sensor. Those are known for randomly dropping the signal and causing all sorts of issues.

:horn:

I have an update! After logging hours of flight recorder data, it turns out multiple sensors drop out for 1-3 seconds. The truck never sputters or bucks, but it is enough to set off the CEL. The wiring harness was checked so it seems to indicate that the computer is acting weird. My goal for tomorrow is to get at the computer and verify the part number, then hope to get lucky at a junk yard.