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Downstream oxygen sensor probes represent significant exhaust blockage and cause exhaust stream turbulence in narrow exhaust pipe. Downstream sensors serve to monitor when catalyst levels fall below certain efficiency. Spark plug anti-fouling spacers should solve this. 1995-2000 LS400 post-catalytic oxygen (downstream) sensor. Right and left bank sensor probes extend into the narrow neck of the front Y-pipe creating turbulence and blockage. To perform testing, acquired Dorman # 42009. This is not tapered version, so it correctly mates with Y-Pipe oxygen sensor bungs and 02 sensor.
Extracted one of the downstream oxygen sensors to ascertain if additional machining is required or if another type of spacer required. Goal is not to eliminate downstream sensor function (only monitors catalytic converter function) but eliminate blockage from exhaust pipes.
Oxygen Sensor spacers bored 1/2" to accept the oxygen sensor probes. There appears adequate clearance around the sensor wire lead end and chassis.
Rear oxygen sensor screwed into modified spacer. Notice probe does not extend past end of spacer, so no interference with exhaust flow. Image depicts oxygen sensor probe inside extender...just enough clearance for expansion and gas circulation. On LS400, downstream oxygen sensors do not instruct ECU on air/fuel mixture. Their primary purpose is to signal when converters are operating below set thresholds.ie; when they are failing.. Downstream oxygen sensor and extender installed. Clearance appears adequate...
PB Blaster applied several times over a week to insure the sensor would loosen with ease. Another angle. OEM oxygen sensor cable is fine with extender
Am not running before/after dyno testing.
Will leave dyno testing to anyone pursuing this project and requiring hard data.
Best guesstimate is sensor probe represents at least 10% restriction....and this is without considering turbulent airflow..
Anyone want to work the math?
It is a budgetary plug-and-play proposition without modifying stock exhaust system.
There is an ECU tech advertising he can disable the downstream oxygen sensor function entirely. n
No trouble code displays. but depending on sophistication of emissions test may show no downstream sensors....
Improved acceleration and the engine pulls much harder at higher RPM''s and not as "winded".
60MPH to 100MPH feels faster.
I have an LS430 (3UZ-FE 4.3 liter) transplanted into my 1999 LS400 (4.0 liter), so alleviating sensor probes obstruction allows the larger engine to exhaust easier in an already restrictive configuration.
I think that modification has the effect of changing the AFRs. If you take a wideband reading before and after, you will see the difference. That could account for the difference in perceived power. You could try to log your fuel economy. It could be running more rich now. Since you have a bigger motor than stock, perhaps a richer AFR brought you to a better state of tune?
Before/after wideband readings were not acquired.
Perhaps the upstream oxygen sensors will adjust to removal of exhaust obstruction.
For before/after comparison of performance with stock engine and exhaust, someone with a 1UZ-FE performing a comparison would present more meaningful results.
Will reset the ECU so engine relearns exhaust with bottleneck removed.
The LS400 ECU is managing 3UZ-FE engine..so far without issue.
Depending on the author, it is reported the ECU can adapt 10%, 15% or 20%...
The ECU does not control the A/F ratio getting the voltage from the downstream oxygen sensor, we need to know. It only detects the efficiency of the CAT.
The function of the upstream & downstream O2 sensors is to measure the oxygen content from the exhaust gas & the health of the catalytic converter, respectively. Not having the tip of the sensor sticking out completely into the gas stream may inhibiting the O2 sensor from doing its job. As a result, it may incorrectly reporting the info to the ECU & worst yet, producing a CEL code. Anyway, it's your car so you're entitle to do any mods but imo, you should run a baseline between the before & after so you have hard data to evaluate & determine whether it's worth it or not.
The function of the upstream & downstream O2 sensors is to measure the oxygen content from the exhaust gas & the health of the catalytic converter, respectively. Not having the tip of the sensor sticking out completely into the gas stream may inhibiting the O2 sensor from doing its job. As a result, it may incorrectly reporting the info to the ECU & worst yet, producing a CEL code. Anyway, it's your car so you're entitle to do any mods but imo, you should run a baseline between the before & after so you have hard data to evaluate & determine whether it's worth it or not.
Here. The upstream oxygen sensors (located in exhaust manifold) were left intact. These measure unburnt hydrocarbons emanating from the engine and informs the ECU to enrichen or lean the mixture based on set parameters...
The downstream (post-catalytic) sensors are not measuring the same gasses as coming out of the exhaust manifold because the catalytic converter has catalyzed those unburnt hydrocarbons into water vapor and carbon dioxide...
What the downstream post-catalytic converter sensor are looking for is measurable hydrocarbons not converted to water or carbon dioxide. If unburnt hydrocarbons at downstream sensors reach a predetermined threshold, then the ECU signals a trouble code.
This explains why no trouble codes in this scenario...because the downstream sensors are not detecting offending post-catalytic gasses.
Newer models utilize the downstream oxygen sensors to instruct the ECU....
On the LS400, from a fuel management perspective, they do not, thus installing extenders to remove probes out of the exhaust stream does not trip trouble codes.
It would be interesting to learn which overseas exhaust systems (Celsior) dispensed with downstream oxygen sensors...
An ECU reprogrammer I contacted is able to significantly reprogram an LS430 ECU, but is only able to modify the LS400 ECU to deactivate downstream oxygen sensors (and ECU security keycode) suggests that in other parts of the world downstream sensors may not be used.(Anyone know?)
The purpose of the downstream O2 sensor is to monitor the amount of harmful exhaust gases flowing into & out of the catalytic converter to ensure the converter is working properly so de-activating/deleting it isn't a good idea.
Ideally, we all prefer not to have any of these emission controlled components, especially the catalytic converter as they're robbing power but they're there for the environment so if your converter is toasted & failed smog, you may not know it in advance if you had de-activated/deleted the downstream O2 sensor.
Last edited by Superfast1; Mar 15, 2021 at 06:59 PM.
The oxygen sensor probe diameter is 12mm = .472" X length 19mm = .748" = .353 square inches.
.353 X 100 = 35.3 ÷2.688 = 13.132% blockage.
13.2% is rounded down to 10%.
Turbulence is not factored in...
The 4.3 liter 3UZ-FE is not quite 10% larger than 4.0 liter 1UZ-FE.
Provided the stock LS400 exhaust system is matched to the 1UZ-FE, then sensor probe relocation suggests the exhaust system flow is now matched to the 3UZ-FE engine.
The oxygen sensor probe diameter is 12mm = .472" X length 19mm = .748" = .353 square inches.
.353 X 100 = 35.3 ÷2.688 = 13.132% blockage.
13.2% is rounded down to 10%.
Turbulence is not factored in...
The 4.3 liter 3UZ-FE is not quite 10% larger than 4.0 liter 1UZ-FE.
Provided the stock LS400 exhaust system is matched to the 1UZ-FE, then sensor probe relocation suggests the exhaust system flow is now matched to the 3UZ-FE engine.
While the math is somewhat accurate the assumption the blockage is static past the point of the sensor is inaccurate.
The protrusion of a sensor does not reduce the overall volume all the way back to the exhaust tip. It is merely a speed bump.
Imo, the improvement if any would be very very negligible. Please prove it otherwise w/ some hard before & after data.
I don't have to prove anything...
If anyone wants to prove it to themselves, which is the only way some here will be convinced, then spend $8 and report your findings on whether 10-13% reduction in exhaust stream blockage makes a difference for you.
While the math is somewhat accurate the assumption the blockage is static past the point of the sensor is inaccurate.
The protrusion of a sensor does not reduce the overall volume all the way back to the exhaust tip. It is merely a speed bump.
It represents some resistance though.
It is not only the sensor probe, but turbulence induced resistance that must be overcome. Exhaust gases ricochet off the wall of match cut tubing sharp angle upon exiting the catalytic converter flange, thereafter is a sharp transition from 60mm to 50mm....and that tumbling exhaust next bounces onto and around the sensor probe. The LS430 exhaust system design dispensed with these issues...
What percent of restriction do you think removing the probe accomplishes?
Trust no one thinks this project increases resistance to exhaust flow.
The purpose of the downstream O2 sensor is to monitor the amount of harmful exhaust gases flowing into & out of the catalytic converter to ensure the converter is working properly so de-activating/deleting it isn't a good idea.
Ideally, we all prefer not to have any of these emission controlled components, especially the catalytic converter as they're robbing power but they're there for the environment so if your converter is toasted & failed smog, you may not know it in advance if you had de-activated/deleted the downstream O2 sensor.
The downstream oxygen sensor does not monitor ....exhaust gasses flowing into the catalytic converter....only what exits the catalytic converter.
The downstream sensors have not been deleted, but simply relocated.
The OEM exhaust system layout from catalytic converter flange to downstream oxygen sensors clearly was not optimized.
If it had received more engineering time, no one would be writing about the design...
Besides power increase, it is my belief relocating the probe will actually reduce emissions and improve fuel economy...
Reducing back pressure, without sacrificing scavenging should decrease blowby vapors the PCV system must work to remove, in the process, reducing PCV vapor deposits on the throttle body and intake valves
Not advocating removal of catalytic converters.
Besides, doing so would only serve to increase exhaust noise.
I added these double stacked antifoulers when i was having a P0420 and P0430 catalyst efficiency codes. After installing both of those antifoulers, I never had another catalyst code again and engine ran/operated fine for the next 40k miles when I sold the car.
Potentially increasing power from repositioning downstream oxygen sensor
