Date: Tue Oct 26 13:42:29 1993   
From: mikef@rosevax.rosemount.com (Michael Foerster)
Subject: Re: High Idle RPM for short periods
To: z-car@uunet.UU.NET@dixie.com

> So does anyone have any ideas as to what the emission controls would be
> attempting to do by keeping the idle speed high for a few seconds after
> coming to rest?  Clearing the plugs?  Running lean to heat up the cat?
> Screwing with the owner's mind?
 
	I know that my 78 has a spring loaded dashpot that does the same
	thing.  When you press the throttle, the pod is filled with air.
	When you let off, it holds the thottle open for a few seconds
	so the throttle doesn't slam shut.

	You should be able to adjust it.

	Mikef

[Modern engines have an Idle Air Control stepper that controls the engine
idle under closed loop control of the ECU by bypassing air around the
throttle.  There is no dashpot.  Comparisons with previous years is 
worthless because emission requirements, vehicle weight and a myrid of
other things change.  If it took a momentary high idle to pass emissions,
that would be incorporated.  

I don't know if the 93 has high idle built in but I do know one thing.
If the IAC is working then the high idle is being controlled by the ECU.
The IAC is pretty easy to diagnoise.  In fact, the ECU should drop an 
error code.  But if the thing is working, one can hear the stepper motor
stepping.  An automotive stethescope might be necessary.  This thing is
responsive to such things as turning the lights on or turning the 
steering wheel, activating the power steering so it is easy to make 
operate at idle.  JGD]

Date: Thu Mar 3 04:01:14 1994   
Subject: Idle air Control
X-Sequence: 3880
From: jgd (John De Armond)

This applies to any car that uses Bosch L-jetronic injection systems
or otherwise use the thermal idle air control valve.  This valve 
supplies extra air to the engine when it is cold.  It consists of a
vane valve operated by a bimetal coil that is heated by both
a 12 volt heater coil and by the engine compartment heat.

My 635 started not idling well when cold.  An examination of the IAC
showed the vane closed even when cold.  The book recommendation is,
of course, to replace the thing, an expensive proposition.  A little
known fact is that this valve can be re-calibrated.  

I first cleaned the thing using aerosol carburator cleaner to make 
the vane was not sticking.  The calibration adjustment is the 
little lock-nutted stud that protrudes from the vane chamber
next to the heater tube.  When the lock-nut is loosened, the stud
slides in a small groove.  This sliding motion operates a cam that
changes the vane position.  

The calibration consists simply of loosening the nut a little, prying
the stud sideways until the vane opens a quarter of an inch or so 
at room temperature and retightening the nut.

Checkout consists of putting the thing in the freezer to make sure it
opens almost all the way and then applying 12 volts to the heater to make
sure it closes fully.

Final calibration involves checking the idle RPM cold vs hot.  The engine
should idle at about the same speed.  If not, the stud can be moved
a bit either way.

This little adjustment saves the ~$100 this valve costs.

John