Quote:
Originally Posted by Paul Workman
According to various sources, the idle air screw is set at the factory and coupled with the TB voltage and IAC module are interactive - especially adjusting the idle air screw. So, it is important to do a diagnostic scan to see that all is well before adjusting anything.
However, TB wear or mods, e.g., porting, headers, or use of reground cams or tuning resulting in altered cam phase changes, etc., may result in a change to the necessary idle speed and consequently adjustment of the Idle air screw and subsequently resetting the TB voltage, etc
If this describes your situation (or perhaps someone else in the same boat), this is (from a tech article I wrote for the HOTB magazine) the procedure that I used to reset the idle air screw on (Ami's) ported and reground cammed 91:
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Setting the idle air screw; the procedure:
- With motor idling at operating temp, note the TPS voltage and the IAC counts and the RPM requested vs. actual RPM.
- Switch ignition OFF. Turn the idle air screw clockwise to reduce IAC counts (to be observed later)
- Turn ignition switch ON, but don't start the motor. Instead, set the TPS voltage to 0.54 volts.
- Cycle the ignition OFF for 10 seconds and then restart the motor to observe the IAC counts.
NOTE: This (4) step procedure is iterative and depending on how much the idle air screw is tweaked will affect the no. of iterations (to be expected). And, on a stock motor, unless there is significant wear on either the TB bushings or throttle plate seat, etc., one would not expect to have to deviate the factory idle air screw setting. So, one might FIRST want to run a scan to see that everything is in proper working order before attempting to cover up a problem!
Hope this helps (someone)! |
Paul,
Unfortunately this procedure does not isolate the Min Air coming from the throttle body itself. The ECM adds and subtracts IAC steps based on a number of parameters which would still be active if setting IAC in this manner. So the proper procedure is to block off the IAC inlet completely. The easiest way to do that is by extending the IAC fully. I described the process for doing that in another thread which I believe Dynomite may have put in the Stickies.
The objective is to set the throttle opening just a tad below where the motor will idle on its own without any intervention from the ECM. The point of good tuning is to have the motor running as well as possible on its own, keeping ECM intervention at a minimum. The closer to optimal the motor runs, the better it will run. The ECM wasn?t designed to make up for a motor running in sub-optimal condition.
I like my idle IAC to be ~ 10-15steps.