Sentra Idle Problem
So I just bought an 04 Sentra 1.8S and I've gotta solve this crazy problem before I put it on the road. It's doing a few nutty things so I'll take it from the top.
It starts with an insanely high idle (~1400 RPM). As the engine warms up, the RPM actually increases! When it hits about 1700, the RPM starts to oscillate in one of two ranges, 1500-3000 or 2000-2500. The range it chooses to oscillate in seems to be random each time the car is started but it doesn't change after the fact. At the top of its oscillation range, the RPM "hangs" for about a second before it starts to come down.
Problem only happens when gear is in park. In all other gears it doesn't oscillate (though the idle is still super high).
MIL does come on but not every time the car is started. It's somewhat random but on average I'd say 1 out of every 5 starts sets the MIL. There is only one DTC, I forget the code but it's description read something like "Idle Air Control System malfunction. RPM higher than expected." - the same code comes back after it's cleared.
I've replaced the entire throttle body assembly twice with known working units and it didn't help, so I think that piece can be safely ruled out.