Ping Tigger: case study on rear O2 sensor and fuel trim

CTI Book Page 104 (CarQuest technical Institute)



.... 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.



(SNIP)



Rear fuel trim Experiment



The Guinea pig; 1999 Dodge Grand Caravan Sport. 3.3 Vin G. 168000 miles



We wanted to determine what, if any, input the rear O2 sensor has on fuel
trim. Many vehicles have rear fuel trim PIDs available. Some OEMs have even
documented the range in which the rear O2 can affect total fuel trim. On
this vehicle however, Chrysler specifically states that the sole function of
the rear O2 is for the catalyst monitor. Period! The rear O2 sensor has zero
input on fuel control. The idea behind this experiment is to control the
rear O2 sensor voltage to the minimum and maximum extreme of normal values
for an extended period of time and monitor short term and long term PIDs on
the scan tool. The rear O2 voltage will be controlled with a sensor
simulator.



(Snip) Steve abbreviates:

a.. Drive 1 Normal conditions
a.. B1S1 Switching 100MV and 900MV
b.. B1S2 steady at 600MV
c.. Fuel trim
a.. Shot term FT +4%-4%
b.. Long term +3%
Drive 2

The sensor is set to 120 MV, low end of the range B1S1 O2s is stuck full
rich; except for the few times the engine experienced slight misfires due to
the over-rich condition.

Short-term fuel trim has apparently reached its limit at +25%.
Long term stopped at +22%



Since STFT is fairly fixed its easy (to calculate the total fuel trim) +47%!
NO CONFERMED DTC' WERE SET! We do have the following pending DTCs: P0172
Fuel system rich B1S1; P0170 Fuel trim fault B1S1. Notice the PCM is
ignoring the B1S1 O2S because it is attempting to optimize the catalyst.
This is the PCMs priority

(Note MPG went from 20 MPG to 15)



Drive 3

Thee sensor is set to 850 MV, the high end of the range B1S1 O2s is stuck
dead lean; Ran fine (but couldn't climb a hill)

Short-term fuel trim has apparently reached its limit at -20%.
Long term stopped at -25%



Since STFT is fairly fixed its easy (to calculate the total fuel trim) +47%!
Once again NO CONFERMED DTC' SET! We do have the following pending DTCs:
P0171 Fuel system lean B1S1; P0170 Fuel trim fault B1S1; P0131, B1S1 Shorted
to ground.

Note the codes do not tell your anything about the rear o2 sensor?





(Note MPG went from 20 MPG to 25)



Anyway, that sums up the first case; I had to abbreviate some stuff
(pictures and graphs and stuff) but it gives you something to think on.

There is another case about a Ford escape; perhaps someday I'll type it up.