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2006 Forester STi SG9 6S M/T
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137 Posts
Discussion Starter #1
Hi,
i just datalogged my forester and realized A/F learning & A/F correction values were always 0.00 without any change.
Then noticed A/F ratio was 14.70 and didnt move from that.

What does it mean? is my O2 sensor faulty? What is the potencial damage if running the car that way?

Honestly I was expecting a CEL to be triggered on but nothing happened juts realized when dataloging.

cheers!
 

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2006 Forester STi SG9 6S M/T
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137 Posts
Discussion Starter #3
romraider..

ok, I know my stock O2 sensor is a "wideband" one but the cheapest and less accurate as Subaru use to install (mine is a DBW forester Sti).
But even that way, during idle A/F values used to change pretty fast between 13.9-14.8 but now is dead and only shows 14.7 all the time without any change.
 

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2008 FSXT M/T
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4,167 Posts
its actually considered a "narrow band" and is not an accurate reading of afr. if your o2 was going bad or dead youd throw check engine lights and your gas mileage would really suffer just for starters...
 

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2017 VW Golf SportWagen 5MT
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10,784 Posts
AFRs in my log using v2 AP do change, something may be off.

Stan
 

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2004 Subaru FXT MT
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1,841 Posts
actually, the front o2 sensor is a semi-wideband, and it is worthwhile. Even narrowbands close or at stoich are extremely accurate, but of course, away from stoich they are awful...

The front o2 sensor can be relied on for accurate reading at low and mid range load. If you relocated it to the downpipe, you can use it as you would a wideband, with a slightly truncated range.

To the OP's questions, under normal cruising conditions, your AFR's should be 14.7:1. I'll suggest that you get a couple of good books and read up on how the internal combustion engine works and study up on thermodynamics, which would cover AFR's.

At cruise and idle, despite the fuel, your ECU is targetting lamda=1. Lamda is different for each fuel. It happens to be 14.7:1 for pump gas.

If your o2 sensor was faulty, it would be pegged at one of the extremes, either reading 20:1 or 11:1, and your AFR trims would be pegged at the limits too, reading ±15%, depending on if the sensor was open or shorted.
 

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2006 Forester STi SG9 6S M/T
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137 Posts
Discussion Starter #7
....If your o2 sensor was faulty, it would be pegged at one of the extremes, either reading 20:1 or 11:1, and your AFR trims would be pegged at the limits too, reading ±15%, depending on if the sensor was open or shorted.
thats what I want to know...
I repeated the test today and checked my former logs and it is obvious that AFR is steady and frezze in 14.70 for any reason (idle and close loop) as well as A/F learning and A/F correction that are freeze in the 0.00 value (former logs show correct/learn values moving from -15% to +15%)

I will replace my O2 sensor and repeat the log.
 

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The 02 sensor on these seem to die slowly with some weird results. Mines definitely going bad as without it the car pulls smoothly, but with it in i get a cyclic pulsation in power delivery and bogging on initial throttle tip in. Going to replace it this week.
 

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2008 LL Bean (4EAT)
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... A/F ratio was 14.70 and didnt move from that....
... under normal cruising conditions, your AFR's should be 14.7:1. I'll suggest that you get a couple of good books and read up on how the internal combustion engine works and study up on thermodynamics, which would cover AFR's....
The ECU maintains the ideal A/F ratio for complete combustion, called the stoichometric ratio, which is 14.7:1 for gasoline. Look about 2/3 down here:
Stoichiometric air-fuel ratios of common fuels
Gasoline 14.7 : 1
Stoichiometry - Wikipedia, the free encyclopedia
 

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2006 Forester STi SG9 6S M/T
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137 Posts
Discussion Starter #10
The ECU maintains the ideal A/F ratio for complete combustion, called the stoichometric ratio, which is 14.7:1 for gasoline. Look about 2/3 down here:
I know that but its never exactly equal to 14.7....the ecu tries to achieve that value at idle and open loop but always showing a range normaly from 13.9-14.8 (in my case).

cheers!
 

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2006 Forester STi SG9 6S M/T
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Discussion Starter #11
I know that but its never exactly equal to 14.7....the ecu tries to achieve that value at idle and open loop but always showing a range normaly from 13.9-14.8 (in my case).

cheers!
I mean that range during REAL TIME DATALOGGING
 
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