O2 conundrum

[ConstantVelocity]

Exhaust leaks upstream of the sensor can also cause them to indicate lean, and the PCM will add extra fuel in an attempt to compensate which (combined with free O2) cooks the cat. It would need to be a big leak to do this. I recall some years of 3.8s having issues with cracked exhaust manifolds, worth checking out. A missfiring cylinder will do the same thing. High fuel trims + dead cat points to unburned fuel and air getting through.

You could try swapping the injectors bank to bank, and replace the o-rings to rule out a leak there at the same time. Compression test too.

 

[Nathern]

Ok LH LTFT +18
SHFT +20
Was going to do compression test but discovered compression tester broken, Have to get new one
Have included Injector pulse from LH Bank and 1 from RH Bank (note the timescale change)
Have included traces from RH and LH O2 sensors
RH o2sensor trace looks good
LH look like S@#t At first glance I would have said a lot on noise so suspect earth problem. Point to pointed wires all good and earth 0.1 ohms which is lead resistance of meter.
Second Clearer O2 trace is the LH sample magnified
Don't have a capacitive probe so can't do the spark?

 

[Futech]

Ok to me O2 is doing its job. Just to reiterate you did say the car is running fine? Also we are talking about 3.8? No vac leaks, exhaust leaks? Basic checks first. Looking at these diagrams to me it would either have inaudible misfire, EGR issue or leaking injectors. Even possible bad wiring, short etc. Sorry could you just add what engine we are talking about.

 

[Nathern]

Yup 3.8, Checked exhaust manifolds could not detect leak there, Can I check the spark from thr primary side of the coil on these engines, to me it looked like the coil and firing circuit was all in the coil module.

 

[Futech]

Yeah that's the only way to check coils on the primary. It will show you spark duration etc. DO NOT probe the secondary or you will fry your scope. Go through all the things I said to check. See how you go

 

[Brett_jjj]

Make sure the 02 sensor earths on the front of the engine are spotlessly clean and tight. If these 02 sensor earth connections become loose or dirty/corroded, the sensor readings can behave very erratically.

 

[Not_An_Abba_Fan]

The first thing I would have done would be to swap sensors left to right. That would have eliminated a sensor issue if the symptoms didn't change.

Having two different cats will most likely be the issue as an aftermarket cat flows more than the genuine. Your exhaust flow from the left and right banks are different so the signals the O2 sensors read and send will be different.

 

[Nathern]

Well first I would like to thank you guys for all the help. Checked all the compressions was 200 all round except No 1 which was 190. Could not find any vacuum leaks. Had already checked all the wring out so Took a long shot and pulled the cat of the left hand side. I could see what looked like the smallest leak around the bung for the o2 sensor. Rewelded the bung in and refitted the exhaust and the O2 sensor is working perfect and the LTFT are back to where they should be.
It obviously does not take much of a leak to upset the o2 sensor.
Anyway thanks aging the help was much appreciated.

 

[Futech]

Good that you sorted it