This needs to be made a sticky by now.
Issues effecting idle:
CEL (Check Engine Light)
Failing/failed/disfunctional O2 sensors
Exhaust Leaks
Dirty MAF (Mass Air Flow) Sensor
Dirty/Worn IAC (Idle Air Contorl)
Intake leak
EGR (exhaust gas recirculation) issues
-Dirty EGR passages
-Failed EGR vaccuum cyclinder (might have another name)
-Malfucntioning DPFE (??? messures EGR flow) Sensor
Vaccuum leaks
bad air temperture sensor
Bad plugs/wires
Dirty air filter
Bad gas
Gas cap not on tightly (cause fuel pressure loss)
secondaries stuck open (lean condition, too much air)
dirty intake manifolds (rich condition, not enough air)
clogged pre-catalytic convertor
clogged main catalytic convertor
dying alternator and battery
worn serpentine belt
timing chain
transmission problem
last time oil was changed (I have seen sluge pour out of oil pans)
Did I miss anything?
There could be alot issues effecting your idle. I would think since your idle is 300 to 400 it would indicate a fairly large problem. Such as bad plugs or the catalytic convertor. But it always be a compination of many small problems depending on the age or condition of the car.
I would think you would have a CEL. As mentioned, autozone and advanced auto parts will scan the car for free.
