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Hi,
I think your throttle plate, is just a cracks width, too far open?
The IAC has a limited amount of open and closing down range.
If the throttle is propped open the IAC is out of the picture of controlling anything lower but with enough signal it will respond to make the idle higher.
It’s just open enough to be overriding the bottom end. It could be as simple as that!
The stop screw is for preventing the plate, from getting jamb in the bore, as the closing spring pressure into gunk build up.
Very possible that servicing the throttle body and not getting set back shut closed enough is the problem.
The ECU should here the full range of the IAC available to it, so it will hold 750 rpm the whole time, despite the A/C load. Of course, this is according to the vehicle emissions decal on the strut tower that allows a plus and minus tolerance. Probably 50 RPM.
In earlier year LH systems they had a knob on the throttle body and a special wire that got grounded to disable an IAC that had three terminals to move it open and closed.
The knob was for setting a slower than normal idle speed, with again, a fully closed throttle plate!
When the wire was not grounded any longer the IAC raised up the speed immediately and the you knew it had the control,
Newer IACs have only two as its spring loaded as a default to be closed all the time until signaled to open. In your case, this all changed around the 1989 plus years and you didn’t get it.
The Bottom line is, in both new and older LH’s systems throttle plate has to be set as close as possible to “shut!”
The throttle switch to “just click” when that plate barely wiggles “to” open.
It’s best to be looking straight at it to see and any slack movement in the linkage and shafts are eliminated.
There is play or slop allowed is a sense of wobble, but not in a linear direction, between the pulley and the throttle shaft.
The “Heim rod linkage” only sets the PULLEY back away from a stop tab.
It only needs a slight space so its NOT touching to touching up!
This detail goes back to the Continuous Injection Spray (CIS) days when they use a standard micro switch in the open air.
The micro switch is now enclosed and engaged on the throttle shaft itself, for more accuracy and damage!
This TPS is critical to shift programs inside the ECU as it makes power transitions more smoothly!
It’s gives the ECU a heads up to a changing air flow mode coming about!
That long post you see within the current following about ripping an ECU open, is well over my head and my best recommendation is to KISS it!
I believe in “Keeping It Simple, Stupid”. (:-)
Today’s cars are going obsolete faster than they can be driven to be worn out!
Phil
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