Customer previously has already replaced spark plugs, coils, checked compression, replaced Front O2 Sensor, and MAF Sensor.
- STFT +-3%.
- LTFT 0%.
Checked at idle, WOT, and introducing air leak (unplugging brake booster hose) to see how well they detect rich/lean mixtures.
- Front O2.
- Fluctuated above and below reference voltage (2.2V at KOEO) properly. Reacted to WOT by going very low then high.
- Rear O2.
- Stayed mostly at 0.9V (a clue, indicating rich mixture).
- With introduced air leak or WOT, went down to 0.1V, meaning that sensor should be ok.
- Checked fuel presure: 54psi. Normal.
What can cause Rear O2 sensor to detect rich, but Front O2 to not detect anything wrong? Considering that customer has replaced so many things already. Hmm.
- Decided to check EGR Valve
- As soon as I took it off the engine, I noticed piece of something (catalytic converter?) not allowing EGR valve to fully close.
- After removing debris, cleaning the valve, and putting everything back together, engine started to idle smoothly.
Checking live data with a scanner again, now, Rear O2 started showing ~ 0.6V-0.65V (normal), instead of ~0.9V (too much fuel), swaying down to ~ 0.1V at times and back. Fuel trims stayed the same, well within +-10%.
This is not a speed density system (MAP system without MAF sensor), so I don't really know why Rear O2 showed rich, instead of lean condition. I'm also surprised that fuel trims were within normal range. Maybe the scanner was not showing things correctly (Autel MS906TS), but who knows.
Its fixed, which is most important, and I'm on to the next one.