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Old 02-16-2019, 04:04 PM   #1
putrio
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Default A/F Ratio at idle?

Hi All, I just bought a new front o2 sensor as the old one seemed stuck at 20.4ish at idle. The new one, unless I apply throttle, is doing the same thing. It goes down to rich while bringing the RPM up and if I hold the throttle at say 2000rpm the A/F ratio goes back up to that same exact number, 20.4 or so. Is this normal? I never tracked that before the engine rebuild and want to make sure it's not going to fry anything.

Thanks!
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Old 02-16-2019, 04:52 PM   #2
JSR84
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No, that't not normal. Whats done to your STi?
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Old 02-16-2019, 05:02 PM   #3
putrio
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Disclaimer...trying to get it to a point where I can drive it to get a pro tune.

It has a new short block, tgv deletes, secondary air pump deleted, new up pipe (nothing fancy the old one just was rusted and crappy). All of that since it has been out of my garage last 2 years ago. I think there is a leak somewhere because it died after revving it to 2000 and abruptly letting off the throttle and smoke seemed to come out around the intercooler area...
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Old 02-16-2019, 05:51 PM   #4
MortonCharbo
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I had a similar thing happen to me. I had multiple exhaust leak. Primarily the turbo to uppipe junction was leaking the worst and pretty much cleared up the issue when I fixed that.
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Old 02-16-2019, 05:52 PM   #5
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I the new O2 an OEM replacement or aftermarket? Generally, your O2 will display 20.4 when it reaches upper end of its detection range, you'll commonly notice it when decelerating in gear, the engine is just moving air and not injecting fuel.
I have a feeling you may have a wiring issue. Out of curiosity what are your fuel trims reading?
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Old 02-16-2019, 05:59 PM   #6
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Originally Posted by JSR84 View Post
I the new O2 an OEM replacement or aftermarket? Generally, your O2 will display 20.4 when it reaches upper end of its detection range, you'll commonly notice it when decelerating in gear, the engine is just moving air and not injecting fuel.
I have a feeling you may have a wiring issue. Out of curiosity what are your fuel trims reading?
I bought a cheap one and it wouldn’t read anything so I ordered an OEM one that came today and is giving me the posted results. I have checked the grounds and the ribbons near the sensor and I think that’s ok...

Regarding the exhaust leaks, I checked all of the pre turbo exhaust and it is all quite tight but I haven’t done an actual smoke/leak test yet.

Edit - Forgot the fuel trims question JSR84 asked...I don't actually know there. What do I need to log/gauge to be able to answer your question?
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Old 02-16-2019, 06:07 PM   #7
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AF Learning 1 and AF Correction 1
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Old 02-16-2019, 06:33 PM   #8
putrio
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AF Learning 1 and AF Correction 1
I ran it for a couple of minutes and AF Learning 1 stayed at 0.00 the entire time, AF Correction 1 was basically going back and forth between 0.00-25.00. The engine would rev a bit and it'd go down to zero and then it'd drop down to idle and it'd go back up to 25. I logged it, if there's a way to get that on here...
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Old 02-16-2019, 07:05 PM   #9
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Originally Posted by putrio View Post
I ran it for a couple of minutes and AF Learning 1 stayed at 0.00 the entire time, AF Correction 1 was basically going back and forth between 0.00-25.00. The engine would rev a bit and it'd go down to zero and then it'd drop down to idle and it'd go back up to 25. I logged it, if there's a way to get that on here...
Makes sense... Your O2 is reporing lean as hell even if its not accurate, so the ECU is trying to compensate with the maximum correction it can (25%). You're probably running extremely rich at idle. No codes?
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Old 02-16-2019, 07:08 PM   #10
putrio
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Makes sense... Your O2 is reporing lean as hell even if its not accurate, so the ECU is trying to compensate with the maximum correction it can (25%). You're probably running extremely rich at idle. No codes?
No codes at all, and you're right, if I have the windows down, I smell like gas even after just a few minutes. I did disable all of the TGV related codes, as well as the secondary air pump codes. Any ideas why the o2 would be reporting so lean when it's correcting to rich? I have tried three sensors now, the original (which I now think wasn't actually bad) the cheapie, and a new OEM...the original and OEM are both reading the same behavior.

I am going to reset learning and log again. Since it's just idling I'll update in a minute with AF Learning A as well.
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Old 02-16-2019, 07:19 PM   #11
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Update - I reset and relogged, exact same behavior and the AF Learning A was zero the whole time.
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Old 02-16-2019, 07:30 PM   #12
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No codes at all, and you're right, if I have the windows down, I smell like gas even after just a few minutes. I did disable all of the TGV related codes, as well as the secondary air pump codes. Any ideas why the o2 would be reporting so lean when it's correcting to rich? I have tried three sensors now, the original (which I now think wasn't actually bad) the cheapie, and a new OEM...the original and OEM are both reading the same behavior.

I am going to reset learning and log again. Since it's just idling I'll update in a minute with AF Learning A as well.
I'm not sure why your 02 isn't reporting correctly. I still think your have an electrical issue. Best of luck.
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Old 02-16-2019, 07:54 PM   #13
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I'm not sure why your 02 isn't reporting correctly. I still think your have an electrical issue. Best of luck.
Do you think it could be a fuse? Thanks for helping.
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Old 02-19-2019, 11:00 PM   #14
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Bumping. Today I started the car and sprayed common leak points with carb cleaner and...well, the intercooler to throttle body seems to have a leak (car revved slightly when sprayed there) and the turbo inlet seemed to have a leak, as well as the turbo to Y pipe connection.

I will fix those as soon as I can, however my understanding of vacuum leaks is that they normally make the car run lean, where my car *reads* lean but is running extremely rich. There is literally fuel collecting in the rolled ends of the exhaust pipes. Exhaust leak? Would it have to be between the front o2 sensor and the engine (i.e. on the crossover exhaust or the manifold), or could it be at the up-pipe turbo junction? Wouldn't the AFR already be measured at that point so a leak would not change the reading, just other symptoms?
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Old 02-20-2019, 02:06 AM   #15
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Leaks allow unmetered air in to the system. The computer doesn't know the air is there.

There are vacuum leaks, exhaust leaks, and boost leaks. They all wreak havoc on readings and the fuel system in slightly different ways.

The sensor is seeing the air there and assuming (reporting) the car is running lean, even if it's not truly running that lean. The computer's response to the sensor reporting the excess air is to dump more fuel in until the sensor reports the proper amount of air and fuel are going through the engine.

The computer can only add so much fuel. That's what you are seeing happen. Chances are, your car is not running nearly as lean as the sensor is reporting, and the fact that the computer is trying to correct it according to the inaccurate readings is causing it to get way too much fuel.

Fix the leaks you know of. Then, with the engine running, take a piece of white paper and hold it near all your exhaust junctions, starting at the headers to the block. You should see pretty easily where your leaks are due to black stuff appearing on the paper.

Let us know what you find at that point.
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Old 02-20-2019, 08:15 AM   #16
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Thank you for that test method - I will try to make time to do it this weekend.
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Old 02-28-2019, 10:28 PM   #17
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IT'S ALLLLIIIIVVVVEEEEEEEE. Purchased new Y pipe hose, throttle body hose, and turbo inlet hose. In the process of installing the blowoff valve hose, I took the throttle body off to make it easier...and found a disconnected hose. I had "clamped" it to nothing. Said about 20x over the next hour "I can't believe there was a hose disconnected". Got everything hooked back up, and the car purred like a kitten. No rough idle, perfect A/F ratios, no fuel smell other than typical exhaust. So flippin excited. Renewed the expired plates this evening, removed garaged status from insurance, and now just need to figure out what's going on with the power steering. I'll make a post for that, I think.
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Old 03-01-2019, 07:12 AM   #18
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Quote:
Originally Posted by putrio View Post
Hi All, I just bought a new front o2 sensor as the old one seemed stuck at 20.4ish at idle. The new one, unless I apply throttle, is doing the same thing. It goes down to rich while bringing the RPM up and if I hold the throttle at say 2000rpm the A/F ratio goes back up to that same exact number, 20.4 or so. Is this normal? I never tracked that before the engine rebuild and want to make sure it's not going to fry anything.

Thanks!
14.5-14.8 is average idle afr
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