Tell me if I am on the right track here... possible bad front O2 or MAF

[thejazzcat]

Hey all,

 

I have been having an issue with my idle AF learning (1A) sitting between 12-14 percent. The car occasionally runs rough at idle. AF ratio is spot on, so I assume 14 is the highest it needed to go to reach stoich balance.

 

Looking in to why, I noticed that the fuel richness may not be needed because of a vacuum leak, and instead may be the MAF or O2 failing and reporting incorrect values. My reasoning behind this is when I did an ECU reset and watched the learning value - the car idled buttery smooth with a 1% fuel addition, but continued to up the fuel level regardless, eventually reaching original learning 1A values of 12-14 again. Idle steadily got WORSE as the 1A value went up.

 

 

To me this sounds like the engine really had all the fuel it needed to begin with, but either the front O2 or MAF sensor is reporting more air than is actually running through the system at idle.

 

The only kink in my theory so far is that I would think you'd see increased learning values across the board if the sensors as a whole were failing...

 

my questions are:

 

-Is it possible for one of those two sensors to fail in such a way that it would only be inaccurate at lower airflow levels (I.e idle)?

-Is one of those sensors responsible for learning values while another is responsible for reporting the overall AFR (meaning the AFR associated sensor is fine but the other is not)?

-Am I thinking along the right lines here, or am I just being a crazy n00b misunderstanding the fueling system?

 

 

Thanks for the input, everyone!

 

thejazzcat

 

 

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[solidxsnake]

Any mods? How many miles on the car? Both the front O2 and the MAF sensors have been known to cause fueling issues and malfunction, but not actually trigger a CEL.

 

First question: O2 sensor shouldn't change with airflow, but it's possible a dirty MAF can report bad values at low airflow, but not at high. Try cleaning it.

 

Second question: no, in essence the MAF and the O2 sensors (along with RPM and anything else the ECU needs to calculate engine load) are the only things involved in learning, and the O2 sensor is the only thing reporting AFR. Meaning your car could think it's actually running at stoichiometric at idle (1.0 lambda, or 14.7 AFR with E0 gasoline), but in reality could be running rich or lean because the sensor is FUBAR, and you wouldn't be able to tell unless you had another O2 sensor in there (i.e. a WBO2 sensor in a separate bung or an external mounted sensor at the tailpipe).

[thejazzcat]

Any mods? How many miles on the car? Both the front O2 and the MAF sensors have been known to cause fueling issues and malfunction, but not actually trigger a CEL.

 

First question: O2 sensor shouldn't change with airflow, but it's possible a dirty MAF can report bad values at low airflow, but not at high. Try cleaning it.

 

Second question: no, in essence the MAF and the O2 sensors (along with RPM and anything else the ECU needs to calculate engine load) are the only things involved in learning, and the O2 sensor is the only thing reporting AFR. Meaning your car could think it's actually running at stoichiometric at idle (1.0 lambda, or 14.7 AFR with E0 gasoline), but in reality could be running rich or lean because the sensor is FUBAR, and you wouldn't be able to tell unless you had another O2 sensor in there (i.e. a WBO2 sensor in a separate bung or an external mounted sensor at the tailpipe).

 

 

 

Thanks for the info! Currently I am running a 2005 LgT stage 2 Cobb OTS w/

- GS uppipe

- invidia long pipe catless with a GESi high flow race cat custom fabricated in to it (single cat setup)

- Borla S-Type cat back

- jdm TGV deletes (codes deleted in tune)

- Avo turbo inlet

- Crawford AOS v2

 

The car runs near perfect at WOT, dynoed at 256/256 whp/wtq with near perfect wideband afr readings from the tailpipe over several runs. It's just idle that sucks noodles :/

 

 

 

 

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[NSFW]

An aftermarket wideband O2 sensor & gauge would tell you what's going on. I'm fond of the PLX gauges (have one in my car) but there are a few to choose from.

 

The MAF sensor tells the ECU how much air the motor is pulling in, so the ECU injects a corresponding amount of fuel (adjusted with the AF learning value), then the O2 sensor tells the ECU whether the mixture is indeed correct, and then the ECU adjusts the learning value accordingly.

 

So a problem with either sensor could lead to the AF learning value going off. A flaky injector could cause a problem too, in theory, but I don't think I've ever heard of an injector causing this sort of problem. Then again, I've never heard of this sort of problem at all.

 

WBO2s are great tools for troubleshooting, so I think they're a good idea for anyone who has a modified car and/or who does their own maintenance. You should get one.

[thejazzcat]

Awesome, thanks for the advice NSFW! When you talk about a Wideband O2, are you talking about a true wideband replacement for the front "kinda-wideband" O2 sensor, or the traditional rear wideband sensor that goes in the second sensor bung on my Invidia downpipe? I assume the latter since I thought one has to keep the stock front/rear O2 sensors in place (since wideband sensors are not designed to interface with the ECU)?

[solidxsnake]

It would be an additional O2 sensor. The front one shouldn't be replaced because, as you said, it is essential for the ECU. People typically either install another bung in the downpipe, or replace the rear O2 sensor and disable its effect on the tune.

 

For the record, I believe the front O2 is just a wideband sensor (not a "kinda" wideband), but because of its location in the exhaust stream, it can't accurately read at low lambdas reliably.