The rear O2 sensor is just there to monitor the catalytic converter and is not used to adjust mixture, so it's not that.
Backfiring the way you describe it is usually unburnt fuel igniting in the exhaust. It can be caused by a mix issue (too lean or too rich can both do it I think), timing that's too retarded, or misfires resulting in unburnt fuel in the exhaust. Also an exhaust leak can introduce additional air into the exhaust which helps ignite the unburnt fuel.
But the bogging and dieseling (run-on) lead me to believe that you either have a misfire issue or mix issue. How is it running otherwise? Smoothly? Have you done a compression test? Any signs of a head gasket issue? How's fuel economy?
Here's what I would do in order of cheapest to most expensive diagnoses:
- You have a DI cassette, right? Check the wiring between the ECU and the DI. Check the wiring to the other sensors as well, esp the MAP and intake air temp sensor. Check for intake air leaks everywhere - replace any degraded vacuum hoses with new silicone.
- Replace the spark plugs if you haven't already with the right NGKs, gapped correctly. Only NGK, only copper. Compare the color of the old plugs to see if any are different from the others.
- Check fuel rail pressure. Replace fuel pressure regulator (if too high) or fuel pump (if too low).
- Clean throttle body. Idle air control valve - check and replace if necessary.
- Replace the MAP sensor. Replace the intake air temp sensor.
- Replace crank position sensor.
- Do the bogging issues happen only when warmed up? Replace the front O2 sensor.
- Clean or replace injectors.
Still having issues? Replace the DI cassette.
posted by 64.134.228...
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