Funny thing, I was in an exhaust shop today getting a quote. We put it on the rack but didn't do any work and I had driven since then for about 10 minutes. I figured maybe a vacuum line or something but it just went away.
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Enable logging for STFT % (short term fuel trim) and LTFT % (long term fuel trim). STFT is the immediate adjustments the car is making to the A/F mixture to either make it more rich or more lean. LTFT is a long term adjustment for the same thing which is stored in KAM - short term is not.
Anyway, start logging while your AFR is bouncing around at idle. If your STFT is high, rev the engine to 2500 rpm and hold it for 30 seconds or so. If the STFT starts to fall to a normal range, you probably have a vacuum leak. Similarly, high LTFT could be an indicator as well. If I recall correctly, LTFT above 10% is considered pretty high. A faulty/dirty sensor, a fuel delivery issue or even a tune issue could also be the cause.
