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O2 conundrum

Nathern

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I thought I had a lazy o2 sensor on my VY II, The left hand o2 sensor was not going full scale as it was oscillating. On closer inspection the LH cat had been replaced so I figured At 150 Km’s most likely the sensor is getting tired or had been damaged when the cat was replaced by the previous owner. So I got a couple of bosh direct fit replacement sensors and replaced them,
Unfortunately the new sensor is doing the same thing. Jumps up and down sharply between 0.6 and 0.3 volts
I put the laptop on the computer and it shows the BLM has moved quite a lot for the LH Bank.
I pulled the injectors and inspected them and they all look clean and ok, cleaned throttle body ect, No change.
The only thing I can see is the LH cat looks a lot smaller than the RH original one and there appears to be a small leak around the base of the bush the o2 sensor screws into.
Anybody else got any ideas Where to look.
 

Nathern

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Fuel is about 10-13 depending on road and it is a little lumpy but nothing I would have been concerned about. Also have just replaced all the leads and plugs.
 

Futech

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Do you have oscillation readings from both O2's? If you can post live scope data of both of them i can tell you whats the problem. Im not after the volt readings at high and low as this would be inconsistent on each oscillation. I would be expecting to see 8 oscillations per 10 secs minimum at around 1500rpm. If you can post these pics i can tell you your issue. You also need to take into account that there may also be issue in wiring, plug etc where it could be losing a very small amount of voltage.
 

Nathern

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Bottom Pix Black is LH orange is RH
Top pix shows l and right long term fuel correction. 0 for RH and 15 for LH
 

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Futech

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Wow they both don't look good. Can you do another measure of them on there own and over 15 seconds Time span
 

Nathern

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Here are the separate traces, don't have much control of time base. Do you have the pin out for the PCM The only one I can find is for a 2 wire pcm and mine is the 4 wire O2 sensors and I would like to put a proper oscilloscope on them
 

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Futech

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Sorry man, i cant read anything with the slow time base. Your scope is to slow as i need to read over 10 seconds to point you in the right direction.
As you have added a new sensor and it is doing "exactly" the same thing all it is telling you is there is a fault. The O2's are working fine. Whats your STFT and LTFT as at a glance we are - and + double digits. So your PCM is trying to compensate for rich or lean depending on these figures. Obviously + lean - rich (STFT) then we can head towards whats actually happening and why. Inaudible misfire, leaking injectors, unmetered vac leak etc etc. Use your scan tool to check STFT and LTFT post those numbers up. Like i said as your scope is not fast enough to record in milliseconds its a bit hard to read. If your images taken were in milliseconds say over ten seconds you would def have a misfire here, even inaudible, slight jagged edge at top of stroke leaking injectors. At a guess looking at this also your car is hunting? How is it driving right now?
 

Nathern

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The Right hand LTFT was approx 2 and STFT was something like -2 so I was ok with the right hand side, the left hand LTFT was +15 and SHFT was equally High (this is from memory) The scope output was from my scan tool, I do have a proper scope ( it will read down to milliseconds easily) I will put on it as soon as I can figure the pin out for the PCM.
I can do it from the connectors but they are in such a [email protected]#$ of a place the PCM seemed much easier.
I had concluded that Due to the LTFT and STFT the RH bank was Ok anf the LH bank had a problem.
Vac leak appeared to only affect LH bank so would have to be a leak not common to the inlet pendulum so could only be at manifold. (Unlikely?) I had checked all hoses and connections ect
Misfire I replaced all leads and plugs with Bosh leads and plugs, Could be a coil I suppose
The driveability seems ok, No hunting that I have noticed and just a little lumpy at idle that I had dismissed as the age of the vehicle and a malfunctioning O2 sensor.
 

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Ok start by doing a hard reset to clear LTFT and run it up again. If you are getting the issue again you would want to start checking for a Vac leak in that bank, post up injector pulse off scope, Ignition spark off scope, and it wouldn't hurt to do things like the basics, compression test to start, burnt valve so everything in that bank is trying to compensate for it. Not saying thats what it is but you have to check basics. Everyone always jumps to sensors straight away before covering the BASICS. When your doing your scope measurements make sure you use your fast one as a "scan tool" is no where near fast enough to catch what your looking for. Take your readings at the O2 sensor plug, don't bother with pinout as your spend to much time trying to find it. I always take my readings direct from sensor, the only time you are heading towards pinout is to check continuity in the wiring. Then afterwards if there was a fault in PCM/ECU. Try these things see how you go. Im assuming you have no fault codes in the system a.
 
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