I did some digging and as ridiculous as it seems ( to me at least) the x300 uses the downstream sensor to monitor fuel and air (a double check) from the upstream sensor data. It would make sense if this is the problem and I simply overlooked it due to them being after the cats.

 

You have never said anything about the history of your car. I’m going to guess that you acquired it with this problem rather than it just happened one day?



I suggested in Post #2 that your O2 sensors are reversed in their connection. I’m still 95% certain this is your issue. You were very dismissive when I suggested that, so let me provide a diagnosis based on the evidence of your data rather than arbitrary suggestions of “it could be this” or “it could be that”.



You clearly have a problem that affects Bank 1 and Bank 2 differently and in opposite ways. Any random suggestions about MAF, TPS, IACV, Relays etc. would not cause that. They are common to both banks so how could they cause such an issue? So you need to look at what things would affect Bank 1 and 2 differently.



Let’s revisit O2 sensor orientation.



Firstly, I will reiterate that it is not possible to determine how the O2 Sensors are setup (orientation) from looking at or testing wiring and tracing from sensors back to the ECU. Internally, the ECU can be programmed to identify sensor A or B as belonging to Bank 1 or 2. So you could have A-1 and B-2, or you could have A-2 and B-1.



If the connections are reversed relative to what the ECU thinks, this is what happens.

Bank 1 goes lean (normal engine running). The O2 Sensor on Bank 1 reacts with voltage change. ECU receives that signal and increases fuel to compensate. But if the sensors are reversed the ECU thinks it’s Bank 2, so fuel is increased on Bank 2 injectors instead of Bank 1.

That makes Bank 2 Rich. The O2 Sensor on Bank 2 reacts with voltage change. The ECU receives that signal and reduces fuel to compensate. But if the sensors are reversed the ECU thinks it’s Bank 1, so fuel is reduced on Bank 1 injectors (which was already too lean) instead of Bank 2.

This adjustment of fueling is the STFT. The ECU is providing the adjustments needed but to the wrong banks. This repeats on each cycle until the maximum rich/lean figure (around +/- 24-25%) is hit on each bank and then ECU cannot do anymore.



This happens only in Closed Loop mode (i.e the ECU is using the O2 sensors to monitor/adjust fueling).



In Open Loop mode (e.g. before the engine is warm, and certain other circumstances) you would not see this condition as the ECU does not use O2 sensor data.



Open Loop is also triggered by hard acceleration (e.g. blipping the throttle). The ECU switches to Open Loop to enable maximum power using its own hard wired fueling map. That is why you will see the STFT reset back to zero. As soon as you stop accelerating, Closed Loop is engaged again and the O2 sensors are used to set the STFT, and the cycle begins again.



Try using your Live data monitor to watch the Open/Closed Loop status while monitoring the STFT and you will see the correlation.



You will probably also see that the +25% and -25% can randomly be on different banks on each engine run. This is just dependent on which one is adjusted first. If you see that, that would be pretty much a guarantee of what I have described as being the issue.





While I’m at it, let’s address the scenario @swisschris had with a “faulty” ECU that caused the symptoms you have.

I’d be willing to bet a large amount of money that first ECU he purchased was not faulty. By bad luck he received an ECU that was programmed the opposite (for O2 Sensors) than his original ECU.

The second ECU he purchased (by good luck) was programmed (for O2 sensors) the same as his original.

So if you are thinking that replacing your ECU will solve your issue, then think again. You have a 50/50 chance that what you buy will be either programmed the same as you have or opposite to what you have. Which do you want? Are you feeling lucky?





Hope this explanation helps someone in the future.

 

This is what happens when you don’t read service info and don’t know what you’re working on.

 

Your extremely correct in the fact that I don't know what I'm working on, hence why I'm here and have been loading the parts cannon.

My mistake for not initially taking the oxygen sensor being switched issue serious, as i watched the voltages drop in their corrisponding bank position on a scan tool and assumed the scan tool didn't "lie"

What would be my best course of action right now? Switch the b1/b2 upstream connections over, reset the ECU and see if the trims work themselves back to normal?

 

And yes I bought the car from an owner who probably should have never touched a car with such pedigree let alone a vehicle in general. These problems have not arrived while in ownership.

 

Here in the UK we only have 2 O2 sensors rather than the 4 that you have, so it's a bit easier.

As your test, you need to swap the connections for the O2 sensors that control fueling.

I only post based on personal experience rather than something I read somewhere. So I can't advise on that as I have no experience of the 4 sensor setup. But from previous posts it would seem to be the downstream sensors rather than upstream that determine fueling in the 4 sensor configuration?

No need to do anything with the ECU. It will reset itself within minutes of seeing the right signals.

 

I switched B1S1 and B2S1 with no change In operation.

I'll switch the 2 other sensors over and see if that helps.

best regards for the input.

 

Yep , I'm armchair quarterbacking and not on the field with experience.

On paper referencing the 801 doc page 81 it only states the downstream sensors for engine regulation and the upstream sensors for Cat efficiency CEL codes 420 and 430

In swapping the 2 aft sensor connectors the engine regulation will be off for a while until the ECU learns the sensors parameters over a period of time and then resume optimal closed loop fuel metering

Your engine runs have probably gone past that point in time

With the 4 O2 sensors you have the option of swapping the aft 2 for the fwd 2 in theory , but in practice the ECU will not recognize the individual O2 sensor behavior and be lost and not " find itself "

So we come back to a ECU orientation with special equipment to tell the ECU the range of the O2 sensors behavior and the ECU is more " matched " and can go forward

In reading someone else in the Boston area he seemed to have observed a difference if the O2 sensor copper crush washers were installed that I don't understand

jagrepair.com/images/Training Guides/801S - 2000.pdf

 

Switched the downstream sensors and the improvement was almost eye-watering.

Still an odd slight bucking feeling at idle but no more stutter, hard start and loss of power. And most importantly !!!THE CHECK ENGINE LIGHT IS GONE!!

Thanks You guys, I'd buy you all a Cold one if you weren't a million miles over the deep blue

 

Happy it’s finally running well for you.

 

Are we basically saying it was what Brendan said at the second post on this thread?

 

I have learned that the biggest problems require the simplest fixes.

 

yes. That turned out to be the winning answer.