Easy there "Blinky". (Can't believe he called you that.)
Thanks. I have an INNOVA, forget which model, along with several others. Find OBD-I harder to work with than OBD-II, at least we ain't counting CEL flashes (are we?).
I have this reader and the extension, very easy to use, and thus I know I have no codes. The IAC has just over 201,000 miles, so I replaced it and got a smooth idle. I disconnected the negative lead from the battery and turned the headlights on for 20 minutes. Turned the headlights off, disconnected the IAC plug, reconnected the battery, had a smooth idle that raced to 1500 rpm, then dropped slowly to 500 and stalled. I turned the throttle screw in 2 turns, idle now at 900, ran for two minutes, shut the ignition off, reconnected the IAC lead turned all accessories on, steady idle at 750. Ran two minutes, shut it off, turned the throttle screw in 3/4 turn, now have smooth idle at 950, with lights and a/c, smooth idle at 800. PS with Houston heat near 100, with a/c on max, engine is running steady at 180. Thanks for the tips.
What you did was to bypass the computer control of the idle by turning the stop screw until you got what you wanted. That's a big no-no; you probably will set code 12 or 13 and won't be able to get the cylinder balance test to run reliably,
Here's the correct way to do it...
Setting the base idle speed:
First of all, the idle needs to be adjusted to where the speed is at or below 600 RPM with the IAC disconnected. If you have a wild cam, you may have to raise this figure 100-150 RPM or so. Then the electrical signal through the IAC can vary the airflow through it under computer control. Remember that the IAC can only add air to increase the base idle speed set by the mechanical adjustment. The 600 RPM base idle speed is what you have after the mechanical adjustment. The IAC increases that speed by supplying more air under computer control to raise the RPM’s to 650-725 RPM’s. This figure will increase if you have a wild cam, and may end up between 800-950 RPM
Remember that changing the mechanical idle speed adjustment changes the TPS setting too.
This isn't the method Ford uses, but it does work. Do not attempt to set the idle speed until you have fixed all the codes and are sure that there are no vacuum leaks.
Disconnect the battery negative terminal and turn the headlights on. Leave the battery negative terminal disconnected for 5 minutes or so. Then turn the headlights off and reconnect the battery. This erases the computer settings that may affect idle performance.
Warm the engine up to operating temperature, place the transmission in neutral, and set the parking brake. Turn off lights, A/C, all unnecessary electrical loads. Disconnect the IAC electrical connector. Remove the SPOUT plug. This will lock the ignition timing so that the computer won't change the spark advance, which changes the idle speed. Note the engine RPM: use the mechanical adjustment screw under the throttle body to raise or lower the RPM until you get the 600 RPM mark +/- 25 RPM. A wild cam may make it necessary to increase the 600 RPM figure to 700 RPM or possibly a little more to get a stable idle speed.
Changing the mechanical adjustment changes the TPS, so you will need to set it. Anything between.6 and 1.0 volt is good. There is no advantage to setting it to .99; that is a BOZO Internet myth, complete with red nose and big floppy shoes.
When you are satisfied with the results, turn off the engine, and re-install the SPOUT and reconnect the IAC. The engine should idle with the range of 650-750 RPM without the A/C on or extra electrical loads. A wild cam may make this figure somewhat higher.
Look in the tech/how to section a sticky thing shows how to do the tps adjustment among other things of interest.
A lot of giys on here, past and present, have put together answers and advice for most all common stang problems and some tips on parts to swap that work.
A lot of giys on here, past and present, have put together answers and advice for most all common stang problems and some tips on parts to swap that work.