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I have a 2002 Subaru Outback 3.0 (6-cyl) with a rough idle.
It evens out at higher RPM.
No codes, but sometimes will read a P0420. Rear O2 performs as expected with WOT test and front AF readings.
Tested both front wideband AF sensors under WOT - behave as expected.
Only suspect PID is a higher LTFT on Bank 1. 10% at times for a total trim of 15% when added to STFT. Bank 2 is much closer to zero. LTFT decreases with throttle
Done so far:
Fuel pressure test per Factory Service manual - passed.
New Spark plugs NGK brand (were due anyway)
Scope of secondary ignition with COP paddle probe and picoscope (Looked fine but very hard to get a good waveform with the COP probe)
Checked for large vacuum leaks with spray bottle
Replaced a few deteriorating vacuum hoses but more remain.
Where should I head next? I am really thinking it's a vacuum leak but this is a speed density engine (MAP only) and I know they tolerate small ones pretty well.
Injector waveform on Bank 1?
Vacuum leak test with smoke machine? (would need to purchase one)
Relative compression test?
Injector flow test?
I would probably go after compression or relative compression testing next to verify the engine is sound. Idle only misfires can be valve sealing related.
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