Setting the 454SS TBI idle speed

10/22/1999BlownSS aka W. Sherwin

Two settings to double check on the TBI are: 1) the min. idle air setting, and 2) the base commanded idle speed and IAC counts. The min. air idle setting (a measure of the uncontrolled air bleed past the closed throttle blades) should cause a warm idle of 625 ± 25 RPM, when the IAC is fully closed and disconnected, the trans is in DRIVE, and the vehicle is held against the foot brake.

Okay, how do you make sure it's fully closed? Is this its position at part throttle when warm?

  • Set park brake and block drive wheels.
  • Start engine and warm to about 190° F.
  • Stop engine.
  • With IAC valve connected, ground the Diagnostic Link Terminal at the ALDL connector under the dash. You ground terminal A to terminal B.
  • Turn the ignition to RUN, but do not start the engine. Wait at least 10 seconds, and the pintle will drive itself fully into the seat in the TB, effectively closing the IAC passageway.
  • With the ignition still in RUN, and with the terminals grounded as above, disconnect the connector from the IAC valve itself. Try to pull the connector "straight" off, and quickly. It is important to break all of the IAC valve circuits at once, otherwise the pintle may move. Alternately, you can also use a Service IAC controller tool to manually close the IAC valve.
  • Remove the ground you installed from the ALDL terminals.
  • These events will have eliminated the IAC from the system.
  • Start the engine, and allow it to idle in PARK or NEUTRAL until the idle stabilizes.
  • Shift it to DRIVE and hold it against the foot brake.
  • If the min. idle (625 ± 25 RPM) is not within specs (altitude also has an effect here) then you'll have to remove the cover plug that hides the linkage stop screw and ever so slightly adjust the blades either more open or closed to achieve the desired results. Drill a small pilot hole in the cover plug, and then use a small awl to pry/rotate it out.
  • When done, shut the engine off, and reconnect the IAC connector.
  • If needed, reset the IAC per the procedure outlined below.

    Sometimes the IAC gets confused about where it is. To reset it, follow this procedure:

    • With engine warm, disconnect the negative battery cable for 15-20 seconds.
    • Reconnect the negative battery cable.
    • Restart the engine for 5 seconds.
    • Key off for 10 seconds.
    • Restart engine.

    The IAC will have been forced to extend fully and then retract to a park position. The IAC should move to a new/proper start position based upon the start-up coolant temperature.

    You will have also cleared any learned IAC modifiers and the IAC system will begin to hunt down the proper idle as run time progresses. It's sometimes good to have an assistant control the key while you watch the IAC in motion, just to make sure it moves smoothly and properly.

Elsewhere

  1. Facebook