There is a difference between Base Idle and Curb Idle.
The idle speed determined by the throttle lever setting on the carburetor or throttle body while the idle speed control (ISC) motor, or any other computer-controlled idle speed control device, is fully retracted and disconnected.
Normal idle rpm. Computer controlled on many modern vehicles.
Ford manually sets the Base Idle at the factory and the Curb Idle is set in the EEC programming. Curb Idle is not adjustable in EFI as it is controlled by the Powertrain Control Module. One should not alter the Minimum Air Flow expected by the computer unless there are significant modifications or someone has mis-adjusted the Throttle Plate Set Screw - and then only after disconnecting the IAC, adjusting to below Curb Idle, reconnecting the IAC, confirming proper TPS voltage and finally deleting the Keep Alive Memory
In other words, the EEC will command a certain airflow for a specific rpm that it is trying to achieve. It then monitors the rpm to see if it's correct. If not, the EEC will adjust airflow up or down to hone in on the target idle rpm. If the IAC Airflow Function and Throttle Body Airflow Scalar values are not what the engine requires, the EEC can start hunting the idle up and down in an attempt to get the idle speed to the target value.
This is not to rule out the many other possible causes of a poor idle, such as:
- Vacuum Leaks
- Ignition Components
- Engine Timing
- Fuel System
- EGR System
- Thermactor System
This brings us to "holes in the throttle plates". Fords original use of the Throttle Air Bypass Spacer Plate was due to warranty concerns of carbon build-up reducing Minimum Air Flow. They later redesigned the Throttle Body Butterfly Plate to include air bypass holes in order to accomplish the same task. The closed Throttle Plate position is communicated to the EEC via the TPS and the EEC expects a specific Minimum Air Flow. With that being said, significant aftermarket engine modifications, such as a lumpy cam, may require additional air flow to maintain a good idle. This is where you see some performance engine builders drilling out their throttle plates. Regardless, those who add or reduce expected Minimum Air Flow without addressing it in the EEC Programming are simply asking for idle problems.
BTW Alvin, if you do go to the Ford Powertrain Control/Emissions Diagnosis Service Manual as the sticker requests, it specifically states "The curb idle and fast idle RPM are controlled by the EEC-IV processor and the idle RPM control device and cannot be adjusted
". Ford cautions to only enter the Idle Setting Procedure after you have eliminated the possible idle problem causes listed below. In other words, an OEM idle requiring adjustment will always be due to one of these causes unless you have modified your engine or mis-adjusted your Idle Set Screw:
- Contamination within the throttle bore
- Contamination within the idle speed control device
- Contaminated or defected EGO/HEGO sensor
- Throttle sticking or binding
- Engine not reaching operating temperature
- Ingnition timing out of specification
- Vacuum air leaks (air intake manifold, vacuum hoses, vacuum reservoirs, power brake booster where applicable, etc.)
On a final note, turning the Throttle Set Screw in a "counter-clockwise" direction closes the throttle plate and each small estimated adjustment must be done with the engine off in order to reset the zero "Ratch" setting of the TPS. Ratch is the output of a ratchet algorithm which continuously seeks the minimum throttle angle corresponding to a CLOSED THROTTLE position.