Here's a way to do it:
Having to set the base idle on an EEC-IV equipped rig is rare because they come set from the factory and they (Ford) say not to mess with it. However, there are times that it is necessary such as when someone replaces the throttle body for whatever reason. Even the companies that manufacture throttle bodies say that they set them up for a proper base idle speed, but that does not always turn out to be true.
Prior to setting the base idle, the throttle body and the IAC must be clean and free of carbon build-up.
Once it has been established that the TB and that IAC are clean and dry, attempt to start the motor. If it will stay running on it's own, great. If not, turn the stop screw on the TB until it will run on it's own. Once you get it to where it will stay running, disconnect the IAC. If the motor dies at this point, the start the motor back up and turn up the stop screw until it will stay running. Now you want to set idle speed via the stop screw to the lowest setting possible between 650-850 rpms with IAC disconnected. It is key to use the lowest possible idle speed at this point to prevent a surging idle once the IAC is hooked back up. Once the lowest possible idle speed is set, shut off the motor and hook up the IAC. With the key on and the engine off, check the TPS voltage. It should be between 0.80 volts and 1 volt. If the TPS reading is outside of that range, don't panic. If the TPS reading is within 0.60 volts and 1.10 volts, it's still acceptable, since no TPS failure codes will be generated, and it falls within the EEC-IV 0.50 volts to 1.19 volts operating range. However, 0.80 volts to 1 volt is the norm. Once a TPS reading has been attained and it is determined that the value is acceptable, disconnect the battery negative cable and leave it disconnected for about 5 minutes. Then re-connect the battery and allow the motor to idle for about 5 minutes to make sure the set-up is operational. If idle speed falls too low or the motor dies, increase idle speed via the stop screw a little bit at a time. Shut the motor off and let it sit for about 1 minute the re-start the motor. Let it run for a few minutes and adjust the stop screw a little bit at a time as needed. If a desirable idle is achieved, then leave the stop screw alone and double check the TPS value. If the idle speed needs more adjustment, repeat the last few steps. If the TPS ends up out of the operational range, the it may need to be adjusted. To do that, you loosen up the fasteners that hold it in place and rotate it one way or the other depending on which direction you need to go. The mounting holes of the TPS may need to be over sized to allow for further adjustment, but this is not usually the case.
Side note: There is no need to reset the EEC-IV since the TPS minimum value used for idle control is reset automatically by the EEC-IV every time the key is cycled on-off with 20 seconds in the on position.
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