O2 and A/F Sensor Readings - I need an armchair diagnosis! HELP!

fitxpert

Hi guys,
I'm posting here in the hopes some of you can help me out.

Car sporadically bogs on acceleration. This gets worse as the car gets warm. Usually it's out of first gear. She's fallen on her face plenty of times at a stop light. Also, accelerating will sometimes have a 2-3 second pause and then sudden acceleration once revs get to 2500+ rpm. Gas mileage has gotten worse by about 2 miles per gallon. The car is not throwing any codes, although through Torque, I did receive a "pending P0420" twice, but neither stuck.

I've cleaned the MAF sensors, cleaned the fuel system with Redline SL1, changed out the air filters, and seen minimal results.

Here's where I'd love your expertise. I have a good friend who is a tech, and he was generous enough to bring a Snap-on OBD Scan tool. We hooked it up, warmed up the car and took some snapshots of what the A/F and O2 sensors are doing. The dealership did the same thing and told me they can't move further because everything is "within spec." I was able to set up Torque so the same sensors are displayed and I took a screenshot.

At idle, we noticed the bank 1 sensor 1 (AF Sensor) undulates up and down (similar to a sine wave), whereas the bank 2 sensor 1 stays pretty flat. What could be causing this? My understanding of AF sensors is that they should remain relatively flat. Secondly the bank 1 sensor 2 (post cat 02 sensor) mimics bank 1 sensor 1 with what looks like a sine wave. Bank 2 sensor 2 tries to stay flat, but sometimes tapers off slowly or drops fast and then rises again. What the heck is going on? I'm primarily interested in Bank 1 Sensor 1. Is the undulation normal? and if not, what could be causing it to do this?

I checked the fuel trims too - Bank 1 undulates and Bank 2 looks like it's relatively steady at idle. What causes the undulation? I'd like to get my gas mileage back to what it was.

Thanks for the input guys.

Sensor Screenshot

Plasmite

I'm new to this stuff, but how is your overall voltage from alternator and so forth?

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WISHLIST

fitxpert

Voltage reads good. I think it's around 12V when engine is not running, and around 14.2V with engine running.

Plasmite

Well you can rule out faulty wiring....

Guess next steps would be to check the housing to make sure vacuum and intake is good.

Most people who witness bogging say it's heat soak. I am not sure of this myself. I have K&N Typhoons and never had an issue with heat soak while others swear their intake air was really high with short pipes. I check my heat shields after a good run (3 hours on highway) and they are nice and cool while my engine bay is soaked.

Seems there are a thousand different causes/effects. Hope you figure it out

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WISHLIST

L33T Z34

I assume the screen shots are from the Z sitting in the drive way logging idle and then revving to 2,500 RPM? Bank 2 (driver side) holds on the rich side per the screen shot. Funny your OBD2 code P0420 is for Bank 1, yet your log shows Bank 2 w/the flat line. Both banks are probably suffering rich conditions at various times.

Do you have aftermarket cats/headers/exhaust installed? Service manual gives the following for possible causes: three way cat bad (leaks), exhaust manifold leak, intake air leak, fuel injector(s) bad, spark plug(s) fouled, improper ignition timing.

Did you say you bought your Z used? Perhaps the original owner had test pipes or hi flow cats and just threw the oem cats back on w/cheap gaskets and/or that red rtv shi+.

I would go about solving this problem by doing these steps:
1) Ensure there's no intake leaks
2) Check spark plugs for fouling/change to new plugs
3) Ensure the oem cat "ring gaskets" & "exhaust front tube gaskets" are replaced w/oem new ones and the oem hardware: two studs, two nuts and the "demon bolt" are good/replaced. The demon bolt usually snaps when removed, so inspect to see what yours looks like.
4) Ensure the exhaust manifold gaskets aren't leaking
5) Testing the fuel injectors
6) ECU ignition timing has been changed (probably the least likely)

If you want the dealer to fix this, you're going to have to get it into limp mode. You should get codes that stick then.

fitxpert

The screenshots are from the car only at idle. I found some lit that states sensor 1 regulates to stoichiometry from the signal received from sensor 2. That's why we see them mirroring each other. To me, it says the cat is not doing its job. Bank 1 is behaving as it should with a switch here and there, Bank 2 is throwing misbehaving. I looked at the fuel trims too. Short term for bank 2 goes up and down just like the sensor readings. Short term for bank 1 fluctuates very little.

Everything (almost) is still stock. I took out the oem air filters and put in K&Ns - then did an ECU reset and had been driving around with that. Gas mileage remained mediocre. I then put the OEMs back in..drove around for a week...no change. So now the K&Ns are back in and I've consistently thrown a pending P0420 code every time I drive. Then, today P2A00. Both pending, I can never get the codes to stick. They always point to bank 2 though.

I have a few PMs in to see if I can score a used cat and just replace it in order to just avoid the dealer.

Thanks for all of your input guys! Much appreciated as I'm trying to wrap my head around this and figure it out. I've definitely learned a ton already.

fitxpert

Just an update. I brought the car in to have them hook it up to CONSULT and test everything and was told nothing was wrong with the car. This was after we had it hooked to the Modis and determined the catalytic converter was going bad.

A few weeks went by and I ended up having code P2A00 stick in May. This was really just the beginning of my problems. The dealer replaced the AFR sensor, because they finally were able to pull the code, but weren't willing to replace the catalytic converter. Several weeks later, same code, and another AFR sensor along with a new intake manifold because the one they put in previously seized into the manifold... A few months went by, and then, boom there's the same code again.

I finally called Nissan Corporate. We're headed into Federal Lemon Law territory...

The light stayed on for about 6 weeks, and the Sunday before my Tuesday meeting with the Engineer sent by Nissan - it went off. The car did something really weird, it had a slow start and my headunit's screen was flashing as if it were shorting out. I got a little worried, but figured we would be able to pull the code at my appointment.

Anyway - at the meeting, the engineer (who looked exactly like Heisenberg, and was a really cool guy) hooked up his CONSULT and low and behold couldn't find any evidence of a stored P2A00. My saving grace was the Low Battery code that was stored. I told him what happened, and he said it probably reset. We went on our merry way and drove around for a good hour, and of course, I almost couldn't replicate the symptoms. It was just way too cool outside.

When we got back to the dealership, Heisenberg gave his recommendations...and in a nutshell, told them I was right all along. He said the catalytic converter wasn't working worth a darn, and that the Z has to be treated as if it has two 3 cylinder engines. He advised replacing the catalytic converter, both rear O2 sensors, and the AFR sensor that was not replaced.

Further, he recommend they replace the CSC as well, because I was still losing fluid after having one instance of clutch pedal sticking to the floor, and replacing the clutch fluid with GTR fluid.

Hopefully, when I get her back in a few days, she'll be good as new and I can move forward with the original "plan."

Thanks everyone for chiming in, and I hope this helps others down the road.

GuiLLoZ

any update on this adventure...?

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Stillen gen 3 CAI/koyorad oilcooler/custom full exhaust/ecutek/20mm spacers/megan springs