Over the last several months my 2000 ES300 (98,000 miles) is accelerating poorly. No check engine fault codes, but two sensors won't report ready: catalyst & EVAP system (report incomplete) no matter how many miles I drive (over 200) and following the protocol to set the sensors - no good. It's been months this way. Engine idles excellent. Starts immediately, but after warm up, acceleration (on-ramp, from stop light) chugs.

I did note that bank1 (close to firewall?) short term fuel trim swings wildly from -15-20% to +15-20%. The other bank O2 sensor is +/- 3% (see picture - orange/red carrots show min max). And the O2 fuel ratio sensor for that same bank also swings wildly from 1.2 - 5.2 while the other sensor is very steady between 3.1 - 3.5. This happens while the car is at constant rpm.

Question: Could my O2 sensor on bank 1 be faulty? - but not throw a fault code? Or is there something wrong with a fuel injector on bank 1?
what else could be going on? I have already tried tankfuls of Chevron fuel system cleaner (and I use Shell Nitro gasoline) which made no difference.

 

I'm not sure about the O2 sensors but me personally I'd check for any vacuum leaks and check your air duct system from the air filter box towards the throttle body. There could be a hole from the air duct system that is bypassing the MAF so its sucking in hot engine bay air when the car is warmed up.

 

On a hunch - and a lot of back breaking fiddling by the firewall - I replaced the bank 1 O2 sensor with an old used one I had on hand that was working when I replaced it (A routine I did at 60,000 miles). I was just curious if the readings would be any different.

Wow....were they. Bank 1 is now rock solid at 3.0-3.2. My short term fuel trim went to zero +/- a few percent - and most importantly - my acceleration is back. Car runs like a top again. All because of a faulty O2 sensor that did not throw a fault code. If it wasn't for the detailed data coming off my OBD2 computer, I would never have detected this.

The big test will be after 100 miles if my incomplete catalyst and evap sensors complete. Then the car will pass inspection later this year.

What a learning experience.

(The rubber intakes are all new)

 

Final notes:

After six months - finally solved this problem. This morning I ran the evap readiness monitor test following the procedure outlined in the service notes. After following the procedure (overnight cold soak, warm up, race engine at 3000 rpm for 10 seconds, then let idle with A/C on), the evap readiness changed from incomplete to complete after only 15 minutes. All readiness monitors are now complete including the catalyst monitor and the engine is now running like a top with full - dependable - acceleration.

All of the issues I was experiencing due to a faulty relatively new Denso air/fuel ratio sensor on bank 1 which did not throw any fault codes. Only by using a high quality OBD2 scanning tool and noticing that the voltage on bank 1's sensor was swinging wildly (1.2 - 4.5) leading to wild swings of the short term fuel trims did I begin to suspect the 02 sensor. I suspected everything else first (bad fuel injector, coil, leak in vacuum, etc.) because the sensor only had 10,000 miles on it - and without fault codes. I am glad I did not just start replacing parts, but kept thinking about the problem.

It was the data that led to the correct diagnosis - and interestingly how the readiness monitors were affected without a fault code.

It was a faulty, but still working 02 sensor.

I hope this insight is useful to others.

(p.s. I did take the car to a shop a few months ago to show them the data and they wanted to sell me a fuel injection cleaning service and replace the catalytic converters - lesson learned on that point too)