Hey, I'm with you on this one too. I have no clue as to what causes this too. When I did the BGR's flip air intake box mod with a K&N filter upgrade, my idle was really weird too. It goes up and down when different things are powered on and off and even braking. The idle change in rpm is subtle, like +/- 100-200 rpms. But this subtle change of -200 rpms at 700 brings you to 500 rpms and the engine wants to quit.
The things that seem to affect the rpms is air, AC, in/out of gear. Once I was on the freeway with the AC on and brake to a stop in stop and go traffice and the rpm dropped to 500 rpm and the engine died. I turned off the AC and cranked it over in neutral and it was ok again.
However, I have since put in a DP... and no change in the fluctuating idle thing. Then put in a larger TB and the idling is much smoother now. Rpms still go up and down but when it dips to 600 rpm, the engine is smooth and doesn't seem like it wants to die anymore. Anyway, I still wasn't satisfied so I backed out the idle speed screw on the top of the TB until my RPM says 700 when I'm in gear with the AC on, lol.
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