This is a LOW idle causation page, or engine Stalls  
A good HOT 800 RPM, regulated , does not happen, if you have other more causative failures, 
Idle is the last thing to  function correctly, or to fix/fiddle  or worry !
Rule 1: The ECU does all it can do, to prevent stalls. (it opens the ISC and advances timing to prevent stalls)
Rule 2: The ECU sets hot (controls) Idle speed, 800 ± 50 at all , times, the idle screw only sets the idle duty cycle , not idle speed. (with A/C on or P/S overload, the RPM is 200 RPM more) {89/90 add defrost and heat blower}
If the engine runs bad, it is quite normal for the ISC to lose control. (it's a servo and is not all powerful; it has limits)
If the engine misfires, bogs, floods, runs rough, smokes black, or shacks this is a bad running engine, and you should not try to correct low idle, or set duty cycle.

Common failures and tests:  (a scan tool sees all this, use  one?  and see closed loop (O2) failing at hot idle (BINGO; fix that first)
  1. EGR stuck open all time  ! or sticking at each traffic stop. (very vary common failure and engine shakes and 400 RPM idle , a  classic and common failure) Clean the main valve, and its fixed.
  2. Dead cylinders are gross MISFIRING.      Weak Combustion?
  3. Forgot to change out the cam timing belt at 60k miles (oops)
  4. Weak fueling ,  lean, bogging, low fuel pressure
  5. Clogged up injector.  (or leaking)  Injector has many failure modes
  6. ISC stuck closed (clean it)  
  7. The 16 valve MPI engine, any un-metered air leaks , can cause a lean fuel mix and bog and run slow.
  8. Bad spark,  or retarded timing due to cam belt slip above. (cam  drives spark so if cam timed wrong makes spark the same) Ever heard of the 60,000  mile interval  tuneup?
  9. ECU stuck in Failsafe mode  (no idle controls) and is so rich , it's doing #2 above , misfiring badly, or is actually in BACKUP mode (way worse than Failsafe mode).
  10. The scan tool or  the dash cluster CEL lamp glowing, running?, means the  ECU  is warning of catastrophic errors. Get the Flash codes or SCAN it.
  11. The TPS idle switch bad or calibrated wrong, will only cause out of control  idle regulation speeds. 
  12. Worse a bad engine, compression low on 1 or more cylinders, if all are low,  then the cam belt slipped, per step 3 above.
  13. Bad fuel? (drain a bit and check it for  contamination signs, of water, rust, bad smell,  bad colors, milky, let it set for 1hr in a plastic jar and see if water settles out)
  14. Other than bad fuel , it's the old classice bad engine, bad spark, bad fuel, in that order, can burn good  fuel with low compression, or bad spark, so they are first.
  15. Found water and green? antifreeze on the tip end  of spark a plug?
  16. Engine flooding, will run slow.
Spark plugs look like this?  (I can see they were new, not long ago (sharp edges)  but are carbon fouled , from weak spark or overfueling, (Fails safe mode active)  (EGR stuck open, fuel pressure way too high)
They are fuel soaked too. (not oil) oil soaking is way more serious.

rev 4 ,  4-1-2014