[cliffyk]

Hello all,

I have seen a number of posts from people having idle problems after installing aftermarket throttle bodies, some to the extent where they said "screw it" and sent it back.

Each manufacturer of aftermarket TBs claims that the proper base idle screw setting has been set "at the factory", however the number of real world idle problems after installing a 3rd party TB makes me wonder.

In any event, I have added a page to my website that provides the proper procedure for setting/resetting the idle stop screw to the proper base setting, and includes some direction for further tweaking of the base setting.

A few people I have provided this to have reported that it fixed their idle problems. I hope it has some value for others.

Clickee here to see...

[01gtMustang132]

Set between 8-900 rpm...
Put a stronger spring on the throttle valve so it will close all the way...
It will give a more positive feel to the throttle also...

[cliffyk]

The PCM controls the idle speed to be whatever it has been programmed to believe is "right", and controls it via the duty cycle of the signal sent to the IAC. Tweaking the idle stop screw sets the fixed air bleed though the closed throttle to a level that is in harmony with the PCM/IAC control range.

Adding a stiffer return spring is a kludge, backing off the idle stop screw 1/16th a turn would have accomplished the same thing.

This notion is the root of the problem people have when attempting to correct idle speed problems with aftermarket TBs.

The idle stop screw on our TBs does not adjust the idle speed in the way that the idle screw on carburetted engines did. What it does is establish a baseline metered air bleed that is actually less air than the engine needs to properly idle. The PCM then adds metered air via the IAC so as to maintain the idle speed at its programmed setting.

If the AC comes on. or the p/s pump or alternator need some help, it extends the duty cycle of the IAC control signal. When the extra power demands go away it backs off.