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Old 08-26-2013, 09:40 AM   #1
whitewater_ncsc
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Default Help me diagnose a fuel trim issue

Hi everyone,

So I have a bone stock 2010 STI (engine wise anyway) and I have been chasing a fuel trim issue where my learned C and D value becomes really high but other trims are fine.

When my learning D value is pegged like this, the car misfires and feels sluggish at high RPM high load like 4th gear 5k rpm +.

Resetting the ECU and the trims will make the car feel great again with no real learned knock but eventually the trims build back up.

What I have done so far :


1) Monitored it over different fuel tank to rule out fuel, at some point I though it was it but it comes back
2) Swapped my MAF with a friend to see if it was an issue with the MAF but no it still did it.


My next steps :

1) Pressure testing my intake, I don't know what kind of leak would give me the trims that I am seeing. Usually, from what I am seeing, A/B trims are the one that get pegged but that's not the case here
2) Swap my front O2. I am really debating doing this before the leak test because it seems easier to replace but I feel bad about just throwing part at the car
3) I have all the HW to go stage 2 but I have been waiting to fix this before doing it. I could slap eveything on as I also got a wideband and this would probably help me diagnose an O2 sensor failure.


Has anyone had a leak that caused just cruise trim to build up but not idle ? I would assume that since my car doesn't knock, my leak is post turbo ? I still build up boost to target so the leak is likely small ?

I attached a learning view and log, any feedback would be appreciated. I depleted all local knowledge (tuner, best dealership etc). Nobody can help me.

Thanks in advance




https://docs.google.com/spreadsheet/...0E&usp=sharing
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Old 08-26-2013, 09:47 AM   #2
dux10
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It is possible to have a small leak that's not really a leak at idle, but with more vacuum place on it, it can become a leak. There's a difference between a boost leak, and post maf leak. A post maf leak can be anywhere after the MAF sensor and could potentially cause these problems. A boost leak is also a post maf leak, but its beyond the turbo. If it leaks between the maf sensor and the turbo, its not a boost leak.

With all of that said, it could be your O2 sensor/s. The rear O2 sensor plays a role in overall fueling correction on the newer cars so keep that in mind.

Steve @ Pullz-On Tuning
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Old 08-26-2013, 11:48 AM   #3
nmlittlebigman
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Are you on a stock tune?
Are you running stock intake?
Do you have an o.e.m. maf sensor installed? (some aftermarkets have been known to cause issues). Also check the o-ring for cracks or tears.

With that extreme fuel trim you will probably hear any leaks post maf (if present) when you do your Boost leak test.

Once you leak test and find/fix any leaks install your wideband to rule in/out front O2 sensor.
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Old 08-26-2013, 12:57 PM   #4
whitewater_ncsc
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Default

Thanks for the prompt reply guys,

As far as the car is concerned :

stock intake, stock maf, stock tune and plugs are maybe 8000km old

So it look like my next step is a leak test. I'd love to hear someone who had similar trim and traced it to a distinct root cause either leak or sensor.

I am still a bit puzzled as to why a vacuum leak would not increase idle trim. My understanding is that the manifold runs higher vacuum at idle than cruise so a leak should impact idle more ? Obviously my understanding must be somewhat flawed, if someone could help correct it that would be awesome too

Thanks
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Old 08-26-2013, 01:48 PM   #5
dux10
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Quote:
Originally Posted by whitewater_ncsc View Post
Thanks for the prompt reply guys,

As far as the car is concerned :

stock intake, stock maf, stock tune and plugs are maybe 8000km old

So it look like my next step is a leak test. I'd love to hear someone who had similar trim and traced it to a distinct root cause either leak or sensor.

I am still a bit puzzled as to why a vacuum leak would not increase idle trim. My understanding is that the manifold runs higher vacuum at idle than cruise so a leak should impact idle more ? Obviously my understanding must be somewhat flawed, if someone could help correct it that would be awesome too

Thanks
vacuum in the intake manifold yes, but airflow through the turbo inlet (between MAF and turbo) increases. This would be where your leak would be.

Steve @ Pullz- On Tuning
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Old 08-26-2013, 05:45 PM   #6
whitewater_ncsc
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Yup okay makes sense, I was thinking about it this afternoon as well.

Fair to say that the leak could be anywhere from the turbo inlet piping all the way to the throttle body (post turbo, intercooler etc) ?

Thanks
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Old 08-27-2013, 12:42 PM   #7
joshuajayg
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I believe he is saying your leak will likely be between the MAF and the turbo. Post turbo leaks under even slight boost will cause your A/F learning to be in the negatives due to less air making it into the combination chambers than the MAF had measured. But before the turbo the air is always under vacuum when you are under load.

Joshua
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Old 08-27-2013, 02:05 PM   #8
whitewater_ncsc
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Thanks,

Good point now I assumed that you would never really be learning in boost since you are mostly in OL. Hence why I didn't rule out a post turbo leak.

Does the ECU stay in CL long enough under boost to learn ? If it is so, cool it narrows thing down
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Old 08-27-2013, 02:13 PM   #9
joshuajayg
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Quote:
Originally Posted by whitewater_ncsc View Post
Thanks,

Good point now I assumed that you would never really be learning in boost since you are mostly in OL. Hence why I didn't rule out a post turbo leak.

Does the ECU stay in CL long enough under boost to learn ? If it is so, cool it narrows thing down

Mmm, good point, I don't know. I'm not that educated on our fueling habits yet.

Joshua
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Old 08-29-2013, 11:25 AM   #10
dux10
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The stock tune will keep you in CL well into boost (up to 4000rpm). This is the entire reason why its so dangerous to have one of these cars untuned...

Steve @ pullz-On Tuning
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Old 08-29-2013, 11:37 AM   #11
joshuajayg
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Quote:
Originally Posted by dux10 View Post
The stock tune will keep you in CL well into boost (up to 4000rpm). This is the entire reason why its so dangerous to have one of these cars untuned...

Steve @ pullz-On Tuning
Darned emission nazis.
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