bigmackloud

Well for posterity sake, I'll post the fix.

After I replaced the factory ECU with no improvement, I decided to take out the EMB completely. Still no improvement.

So I sat down with a pinout of the ECU harness and started checking the voltages pin by pin. The O2 sensor signal still wasn't where I thought it should be.

Two key points here. I previously mentioned 1) I suspected the PO had wired the 4 wire O2 sensor incorrectly. and 2) I ran a new signal wire and a new ground wire.

I did not however check the O2 heater wires. I made the assumption these were right. Classic mistake. I had also assumed that since a single wire O2 sensor is sufficiently heated by the exhaust, and that since my sensor is just below a 1000* turbo, that even if the sensor heater wasn't working correctly, the sensor would still be up to temp from the exhaust. Also a mistake. I'm not sure if the sensor wasn't getting up to temp, or if the sensor simply needs that circuit to be functioning to read accurately. But either way... it makes a big deal.

I found that the PO had wired both of the heater wires to 12V. That's incorrect. One is a 12V and the other is ground to complete the circuit. To be fair, the Bosch instruction don't actually state this, only that there's no polarity. I guess he was assuming the heater got it's ground from the ground wire. Also, both wires had pulled out of his electrical connector, so they weren't even getting the 12V he had intended. This wasn't visible though because he did an excellent job with the electrical tape, haha.

So I corrected the wiring and added my EMB back into the loop. A quick test drive revealed no more random lean condition. I've still got some tweaking to do but hopefully this is behind me and I can move on to what I had originally intended to work on 2 months ago... in-boost tuning.

I think I'm going to replace the O2 sensor again also, just in case it fouled any since it wasn't heating properly.