E46 Fanatics Forum banner

Oxygen Sensor Headache

416 Views 10 Replies 5 Participants Last post by  LeverThis
08/98 323i w/ M52TUB25
Ebay Headers
new Bosch 13477 O2 sensors
EU2 MS42 DME flash (7528736)

I've had headers and EU2 before I started tinkering. The car ran good except the exhaust leak and poor fuel mileage, which I attributed to unmetered exhaust leaking before the precat O2's. My ebay headers had flanges warped to hell and leaked, so I took them off and put them on a Bridgeport to mill the mating flanges flat. New studs and nuts were used on assembly to ensure no leakage. This fixed my initial reasoning why I started this, but a can of worms has been opened.

Upon first startup, car ran with no issue. After a few trips, it started to stumbling and running very rough and lack of any acceleration. Codes were set for "precat O2 min value undershot." Was able to stop and clear codes and adaptations (for s&g's) and get home without issue. To me, classic symptoms of swapped O2's, no problem, swapped them.

Now that O2's are swapped- for first ~3 minutes on startup, the car runs very rough. Feels like very (very) bad misfires and cannot maintain idle, sometimes dies out. Throttle input gives no response, sometimes bogging the engine. After this first ~3 minutes, car runs fine and drives like a bat out of hell.

Diagnostic tools available to me are INPA and ScanMaster-ELM. I know, INPA isn't good for what I am up against, but its what I have in the moment. Initial findings with INPA show (while idling) my fuel trims remaining constant. (Error b/c unplugged, but those are the exact values I was seeing.) I am unsure if there are X number of drive cycles before fuel trims start setting after adaptation reset, but in hindsight, I really shouldn't have reset it.

Rectangle Font Parallel Software Slope

My next step was to look in ScanMaster-ELM. I plotted the precat O2 data and I was stunned. My precats were reading constant (0.022v and 0.025v respectively). If I unplug the MAF to force open loop, I see the sine waveform I would expect, but MAF plugged in I have constant, flat-line readings.

Currently, I am going to go verify that O2's are in correct position by unplugging one and seeing if it loses connection on the software end (engine off of course). I have OBDFusion adapter on the way, but I won't receive it in time before I leave back to school, so this will likely continue into May.
See less See more
1 - 6 of 11 Posts
Where did you buy the O2 sensors? Unless it was a known legitimate source (dealership, FCP, etc.) the internet is flooded with counterfeit parts that can cause the kind of issues you‘re describing.
Amazon and eBay or notorious for such parts.
If you did buy from a legit source that’s one less variable.
Bought from autozone. They were priced better than FCP, although I usually buy through FCP. I’m not worried about the legitimacy of the sensors, but could be both DOA, although unlikely.
Good work on sorting the leaking header flanges.

Sounds like the pre-cats are telling the truth. The mixture is running very lean when the MAF is plugged in. Smoke test the intake to find any vacuum leaks. If non found, then smoke test the exhaust.
I’m 99% sure I’ve accidentally discovered my lean condition.
Motor vehicle Automotive tire Automotive design Automotive wheel system Gas

Motor vehicle Hood Automotive tire Automotive design Automotive exterior

Laying under the car and I look up seeing that hole, thinking hmmmm. It’s odd that now it pops up. I guess that’ll go in my next FCP return shipment. Had an old spare (replaced from the FCP CCV kit) Uro brand one, hopefully it’ll last more than 2 years.

After swapping, I drove the car with literally no issue. Drove great. So far fuel mileage is much higher too. From 14.7 up to 19.5 in only a few miles. I’ll see how long that lasts this summer lol. In the morning I’ll see if rough idle appears, it’s been a consistent occurrence after sitting overnight. I’ll also look if O2 waveforms are proper, maybe ScanMaster-ELM isn’t the right pick for what I’m doing.

Motor vehicle Automotive tire Bumper Automotive exterior Rim

Motor vehicle Wood Tool Automotive design Gas

Wood Floor Shipping box Flooring Automotive tire

Ty for props on the flanges. I had an “oh shit” moment when bank 1 header came loose on the jig, had to weld it up the next day. It’s almost like they should come with this beautiful finish lol.
See less See more
  • Haha
Reactions: 1
It was worth a guess- Autozone seems like it should be safe from knock offs.
Indeed worth a guess, not knocking you for it. I learned my lesson there with a duralast brake light switch that set EGS codes and light.

If Autozone was selling fakes then I’d be worried FCP or all the other reputable places would be victim too.
  • Like
Reactions: 1
To bring closure to this thread, the lower intake boot was my issue. I find it strange that only now it shows itself. I wonder how long it had been torn.

Anyway, coincidences happen.
Get access to a smoke machine to find leaks in vacuum, exhaust, evap systems. Everyone needs one!
I need too, it’s been on my mind for some time. Idle O2 voltages read ~0.055V, but under throttle they oscillate now, so (if I interpret correctly) I still have some lean condition. Also have high positive STFT’s (25-27%), so I feel like that’s more proof toward lean condition. Granted, I’ve only drove it less than 50 miles so that might be an exaggeration of what the STFT’s would converge to.

Poor car needs more time spent with it than I have time for sadly.
1 - 6 of 11 Posts