Idle speed on fuel injected engines is computer-controlled. There are no adjustments. There is an air passage that bypasses the throttle blade. Your engine uses a two-wire spring-loaded solenoid to open the valve that adjusts the amount of air coming in at idle. That passage can be blocked with carbon.
You need a scanner to command the Engine Computer to work that solenoid to see if it responds properly. You can also view live data to see what the computer is seeing and reacting to. One thing that Ford had a real lot of trouble with in the early 1990's was their coolant temperature sensors. Those are normally extremely reliable because they have just one component inside them. Most problems with that circuit come from corroded connector terminals. Ford managed to have a huge rash of failures, but the common symptom was erratic idle speed. Idle speed increases at lower temperatures, and it surged up and down in response to the erratic readings from the defective sensor. The scanner will let you see what the computer is seeing for coolant temperature, and it will display the percent of increase in idle speed it is requesting. That information will tell you which area to start diagnosing.
Tuesday, February 28th, 2017 AT 4:16 PM