Completely stock engine with 74,000 miles on it stumbling between idle and 3,500 RPM

[S1Motorsports]

the studering RPM under load is caused by a bad A/F sensor which directly is affecting fuel injection strategy due to abnormal signal although still within monitor threshold for ecm monitors, so no codes set , had this issue on a 14 GS350 exact same condition , disconnected A/F sensor placing vehicle in open loop fault (thus CEL is illuminated) but it goes to a default map and RPM studder went away , replaced (in this case) both A/F sensors and issue was fixed.

[steve99]

Originally Posted by S1Motorsports

the studering RPM under load is caused by a bad A/F sensor which directly is affecting fuel injection strategy due to abnormal signal although still within monitor threshold for ecm monitors, so no codes set , had this issue on a 14 GS350 exact same condition , disconnected A/F sensor placing vehicle in open loop fault (thus CEL is illuminated) but it goes to a default map and RPM studder went away , replaced (in this case) both A/F sensors and issue was fixed.

Yes its possible bad 02 sonsor cauld cause hesitation under load but so could a bunch of other issues like bad maf sensor dirty maf or condensation on maf, coil packs misfire dont always throw code or light CEL till certian misfire thresholds reached. Could even be knock due poor fuel or bad injectors , fuel pressure issues or di computer not earthed vaccuum or exhaust leak or dozzens of other issues.



Without reviewing logs of issue with extensive list of parameters logged everyone is just guessing.


Would be unfortunate if this guy go and buy a $200 02 sensor only to find its not the problem.

[S1Motorsports]

Originally Posted by steve99

Yes its possible bad 02 sonsor cauld cause hesitation under load but so could a bunch of other issues like bad maf sensor dirty maf or condensation on maf, coil packs misfire dont always throw code or light CEL till certian misfire thresholds reached. Could even be knock due poor fuel or bad injectors , fuel pressure issues or di computer not earthed vaccuum or exhaust leak or dozzens of other issues.



Without reviewing logs of issue with extensive list of parameters logged everyone is just guessing.


Would be unfortunate if this guy go and buy a $200 02 sensor only to find its not the problem.

I said disconnect the Air fuel sensor for the vehicle to go into open loop fault and see whether the symptom goes away You wont have to go out and buy one to see whether the problem would be fixed its a quick check disconnect the A/F drive it , if symptoms goes away get a new one If its still there reconnect the A/F clear the code and continue diagnosing. voila! The O2 sensor is not at fault Its the A/F .

[S1Motorsports]

here are attached of two data files I logged of the condition. first one shows the flutter occur in the RPM parameter second picture shows the result after only disconnecting the A/F sensors in this case B1 and B2. After the result I replaced the sensors and confirmed my findings.

its worth a shot to disconnect and see whether the symptom goes away its free diagnostics lol

[Ultramaroon]

Originally Posted by S1Motorsports

I said disconnect the Air fuel sensor for the vehicle to go into open loop fault and see whether the symptom goes away You wont have to go out and buy one to see whether the problem would be fixed its a quick check disconnect the A/F drive it , if symptoms goes away get a new one If its still there reconnect the A/F clear the code and continue diagnosing. voila! The O2 sensor is not at fault Its the A/F .

I've been trying to think of another fault that would be masked by forcing open-loop operation and I can't think of any.


I'm inclined to believe this is a valid test.

[steve99]

Originally Posted by S1Motorsports

I said disconnect the Air fuel sensor for the vehicle to go into open loop fault and see whether the symptom goes away You wont have to go out and buy one to see whether the problem would be fixed its a quick check disconnect the A/F drive it , if symptoms goes away get a new one If its still there reconnect the A/F clear the code and continue diagnosing. voila! The O2 sensor is not at fault Its the A/F .

Yes i can see your logic.

However disconnecting the 02 sensor is may to send subaru ecu (86/brz same) into limp mode (not just open loop) where i believe it uses default maps and derives it load calculation from rpm and a default table instead of MAF sensor signal. So your not only ignoring the 02 reading your likely ignoring the maf sensor as well. And possibly other sensors, and running extremely conservative default fueling/timing maps and likely not VVT cam control and rpm limited to about 4000. Its possible that in limp mode you may also mask a problem in other areas not just the 02 sensor. this maybe different on toyota ecu logic.

It also possible that the car has developed and exhaust leak which has caused the ecu to use a lot of fuel trim to try and correct a perceived fueling issue, which in turn has cause the rough running issue.
disconnecting the 02 sensor would again mask the exhaust leak issue and the car would likely run ok .

stutter or hesitation could also be caused by poor quality fuel inducing knock and ecu pulling timing, again pulling 02 sensor and running on conservative mapping may mask the poor fuel.

unfortunately these cars also have coil pack issues which can cause misfires/hesitation under load and dont always trip the cel threshold for misfire and also Direct injector seal fails which dont always trip cel and cause hesitation stutter under load.




In your case with the V6 lexus the data on your screen shot appears to show a clue to the issue

Target AFR is 0.997 lambda


AF Lamba B1S1 = 1.2
AF Lamba B2S1 = 1.092

that seems to indicate issue on the Bank 1 afr sensor or possible exhaust leak. as the B2 sensor appears to correlate with target afr but the B1 sensor is reading very lean. assuming that data was in condition with both sensors plugged in ?

[B.Pod]

Hey Pat, did you manage to find what is going wrong on your car ?

Here is another video of hesitation I experienced on my car :

What do you think about this hesitation behaviour ?
I'am not on full throttle but just trying to drive smoothly in the trafic...
The drivability is not alaways as bad as the video example but never smooth like an atmospheric engine shoud be.
No MIL, No clue...

[Pat]

Unfortunately I still don't have a resolution. However, I have not experienced the problem in a couple months. I don't know why that is, but it may be due to cooler ambient temperatures. I'm just waiting for it to reoccur at this point.
Your car seems to exhibit a similar, but I don't think identical, behavior. Mine isn't that consistently bad, and not to the same degree.