What can cause slow oxygen sensor readings?

Question

I have very slow primary (pre-cat) oxygen sensor readings on my 02' Corolla (2ZZ-GE, 141kW N/A).

While trying to investigate the issue, I repeated diagnostics frequently. Before every diagnostic attempt, I fully warmed the engine and raced it @ 2500 RPM for about two minutes.

Output of oxygen sensor readings:

typical 0.1Hz, max. 0.2Hz @ idle (very slow and not constant)
typical 0.5Hz, max. 0.8Hz @ 2500 RPM (extremely slow and not constant)
Graph

Additional and possibly related troubles:

Erratic idle
Engine speed follows oxygen sensor signal
LTFT -15% at @ idle and about 0% on next minimal TPS reading

I have tried the following:

changed to a new OEM oxygen sensor (primary, pre-cat).
changed to a new OEM MAF sensor.
changed to a new OEM PCV valve.
changed to a new set of spark plugs.
changed to a set of used, low millage and cleaned OEM fuel injectors.
cleaned throttle body and idle air control valve.
checked VVTL-i (lift) valve and filter. Working and perfectly clean.
checked for intake air leaks. No were found.
checked for exhaust air leaks. No audible/evident leaks were found.
reset ECU.

No success at all. What else can I try? Thanks :)

Robert Ruggiero · Answer

You stated the O2 in Hz. Typically the O2 moves below and above.5. So, when I am diagnosing I will be looking for it to cross the .5 volt threshold about every 5 seconds. The ECM doe compare post and pre O2's to calculate converter efficiency. I see you replaced many parts;but, not the O2?

Answered by Robert Ruggiero on April 29, 2021

Zaid · Answer

Assuming these are narrowband O2 sensors, the sensors should cycle far quicker than that once they are up to operating temp.

From experience, an ageing O2 sensor will go lazy with time and exhibit the sort of behavior you're observing. In all probability you will have to replace the pre-cat sensors.

This video provides a useful reference of functional vs lazy sensors.

pabx06 · Answer

symptoms: LTFT -15% at @ idle and about 0% on next minimal TPS reading.
What is going on: your injection ecu is reading rich condition and reducing fuel injected when fuel system is in closed loop
hypothesis

Makes me thinks of a tiny leak when injector not closing well/cloged, injector leaking a tiny amount of fuel => when rev up RPM leak amount become proportionally small when high amount injected.

Or fuel pump/feeder pressure outputs too high and injector are fine. and RPM at idle is smooth.

Unlikely but fuel octane in tank is too high for the engine ?

air intake manifold sensor lazy

checks:

the proper clearance of Crankshaft position sensor

And if any camshaft position sensor. And clean them up.
Those can have magnet and they collect metallic debris.

ECU: recheck ECU wire socket for dirt oil rust. and harness you will probably check resistance with multi-meter and read 0-1 ohm

uneven idle/ rough/  Shaking engine/ trouble with firing or EOBD or scope : try to plot the engine speed at idle with high resolution coming from the Crankshaft position sensor if your engine is a 4 cylinders: (couple of cylinder firing/spark ignition) you should see a pair a bump in speed (two waves like when each cylinder fires) you have 3 cylinders that are starving fuel from the leaky cylinder. your speed bump wave will increase differently one small wave and another bigger
when cylinder is rich typical it produce more tork than others hence biger rpm jump when if fires

What can cause slow oxygen sensor readings?

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