Misfire | Subaru Forester
🚗 2011 Subaru Forester 2.5L
- Car came in misfiring on all 4 cylinders at idle.
- Customer previously has already replaced the spark plugs, coils, checked compression, replaced the front O2 Sensor, and the MAF Sensor.
First, I took my diagnostic scanner, went to Engine ➱ Live Data, and started the car.
- STFT +-3%
- LTFT 0%
- Both are well within normal range (+-10%)
Checked O2 sensors live data at idle, WOT, and introducing an 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 (lean) then high (rich).
- Rear O2
- Stayed mostly at 0.9V (a clue, indicating a rich mixture).
- With introduced air leak or WOT, went down to 0.1V, meaning that sensor should be ok.
Checked fuel pressure: 54psi. Normal.
What can cause Rear O2 sensor to detect rich, but Front O2 not detect anything wrong? Especially considering that the customer has replaced so many parts already. Hmm.
- Decided to check EGR Valve
- As soon as I took it off the engine, I noticed a piece of something (catalytic converter?) not allowing the EGR valve to fully close.
After removing debris, cleaning the valve, and putting everything back together, the engine started to idle smoothly.