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You currently have no cat but your secondary O2 sensor is plugged in and you haven't deleted the code? How is that even possible?
i have a long tube header, the downpipe off it has no cat. but it has the rear o2 bung still.
currently my rear o2 is in the secondary bung, its reading the exhaust but the computer expects a cat. so the ltft readings are based on there being a cat there, possibly skewing my air/fuel mixture over time based on those readings (i'm not 100% sure how these ECMs use the rear o2 compared to other EMS systems.
my question was if i would be better off leaving the rear o2 unplugged and the bung plugged off and living with the CEL, or how i am now.
I'm more wondering how you're not getting a MIL for there not being a cat. The main reason for that sensor to exist is to verify that the cat is functioning, so if it's not throwing a code then it's not working properly. I had to do a little reading and i'm also not sure if the rear sensor is used for LTFT on these cars. The sensor should be verifying that there is never a lean condition coming from the cat (because the cat should be using oxygen to complete combustion), so it's possible that the computer is compensating rather than throwing a MIL? Do you not have a way to turn off the CEL if you unplug it? I wouldn't want to run with a constant LTFT because it's either real and something is causing a lean condition, or it's not real and you're burning more fuel than ideal. You're not the first one to run without a cat so others have definitely solved the problem. Your situation sounds like something is wrong.
from my understanding they don't always throw a code with no cat. the LTFT aren't far enough off to throw a code its usually +-9ish iirc it has to be like 20-25 off to throw a code
i dont have a tuner to turn off the rear o2. if i unplug the rear o2 it will have a constant o2 circuit or whatever code.
if that's true then their MIL coding sucks. it should be looking for similarity between the primary and secondary, since the whole point is to see if the thing exists and is functioning.
yeah not sure, i think i will remove LTFT from my gauge screen on torq and just not worry about it since with these engines the 2nd o2 doesn't adjust anything
from my understanding they don't always throw a code with no cat. the LTFT aren't far enough off to throw a code its usually +-9ish iirc it has to be like 20-25 off to throw a code
i dont have a tuner to turn off the rear o2. if i unplug the rear o2 it will have a constant o2 circuit or whatever code.
LTFT is based on the primary, I think it's +-15% to set a code. If your trims where never dialed in to +-3% then a shift to a higher +-value in weather change isn't unheard of.
Your rear O2 should be reading just like the front which should be setting a code after your first 2 drive cycles. The rear O2 after the cat warms up has a steady voltage if the Cat is doing it's job. I unplugged mine and turned off my Post O2 diagnostics... No light and still a perfect ST & LT fuel trim.
Find someone with an HPT that knows how to set fuel trims in open loop. The HPT can force it back into open loop and you can also reset LTFT and then watch the immediate movement.
If they take their time and know what they are doing you can get it dialed in this tight.
the post-cat oxygen sensor has minimal
control over fuel trim under normal operating conditions. GM material states
the rear O2S has +/-1.0% authority over fuel control.
Some cars have 3 pre cat O2's left bank, right bank and y pipe. You are probably thinking of that one. fuel trims cannot be adjusted off of a post cat O2. At least not the fuel trims that the car operates off of daily. It can use what it sees to help protect the cat but that's not considered a fuel trim to meet a 14.7:1 A/F
NOTE: On early OBD II systems, the post catalytic converter O2 sensor has no effect on fuel control. The post catalytic O2 sensor was originally responsible for only monitoring catalytic converter efficiency. On most systems, the post converter O2 sensor signal should never mimic or follow the pre-cat O2 signal. That would indicate a defective or low oxygen storage capability at the converter . On early OBD II systems, the post-cat O2 sensor should show little or no voltage fluctuations on a scope waveform, since all the mixture fluctuations are being absorbed by the catalytic converter.
Stating around model year 1999, a new type of converter came on the market, called “Low Oxygen Storage Converter” or LOC. With an LOC, the pre and post O2 sensors cycle at the same rate. These converters are tested by measuring the lag-time between the two signals. A further development of this system is that the post converter signal is also used for A/F correction, but to a less extent.
I don't get my info from Google. I get it straight from GM engineers that developed the systems.
Like I said... It doesn't use it for trimming A/F to 14.7:1. It uses it to save the cat from being damaged. Yes, it is fuel trimming but not what your concerned about. Your concerned about LTFT which is based off of your primary O2.
That's good..... They should have been more specific in what they were talking about. Do you understand a lot of articles and even college books are written without ever consulting the manufacturer?
What is the purpose of the post cat Oxygen sensor?
--Catalyst efficiency monitor?
That's only a portion of its function
REAR FUEL TRIM
a.. The Post-cat O2S has been used since 1988 to control fuel trim (to
some extent)
a.. Toyota and Saab
b.. Used to fine-tune the A/F ratio to maximize catalyst efficiency
c.. Will also compensate for a degraded Catalyst
d.. Every manufacture today uses the rear O2S to trim fuel today
The post cat O2S is a fuel control input. Toyota and Saab have used the
post-cat O2 to control the fuel trim since 1988; Before OBD-II. This sensor
is used to fine-tune the air-fuel ratio to maximize catalyst efficiency. It
can also adjust the air fuel ratio to compensate for a degraded Catalyst. If
you were to measure the actual amount of time the post-cat O2 is used during
the running of the catalyst monitor (maybe once per trip) and compare that
to the amount of time the post-cat O2 is used fine tune the air-fuel ratio
(almost always in closed loop), you will see that this sensors major
function is fuel control. Every vehicle manufacture today uses the rear O2S
for fuel correction; even if it is undocumented.
a.. How much control over fuel trim does the rear O2S have?
a.. GM material says less than 1.0%
b.. Ford says 0.5%
c.. Toyota says 2.0%
d.. Under normal conditions
b.. We have seen up to 30% correction in fuel trim from the rear O2S in
abnormal conditions
(SNIP)
According to these manufactures, the post-cat oxygen sensor has minimal
control over fuel trim under normal operating conditions. GM material states
the rear O2S has +/-1.0% authority over fuel control. The Ford OBD-II manual
leads us to believe this sensor is of little consequences in regard to
drivability as it only has +/-0.5 fuel control authority. Toyota allows for
a whopping 2.0% +/- correction. However these claims are under normal
driving circumstances. What happens if there is a problem? We have
documented up to 30% correction in fuel trim from the rear O2S in abnormal
conditions.
03-15-2018, 04:28 PM
