If you are experiencing a rough idle on your vehicle after you disconnect a battery for any reasons, you need to perform the idle relearn procedure. Late models of Ford vehicles equip an electronic throttle body (ETB), which is fully controlled by a vehicle computer. An ETB eliminates the need for an idle air control valve and a physical linkage between an accelerator pedal and throttle plate. Instead, the vehicle computer controls all these things by sending a signal to a motor inside the ETB based on the readings from various sensors. When disconnecting a battery, the computer loses communication with the sensors, as well as memories that have been stored in the computer The communication must be re-established by performing the following Ford idle relearn procedure.
Please note that there are a few different procedures depending models and years. The procedure below is common in most 2000’s Ford vehicles.
Because your vehicle’s engine is electronically controlled by a computer, some control conditions are maintained by power from the battery. When the battery is disconnected or a new battery is installed, the engine must relearn its idle and fuel trim strategy for optimum driveability and performance. To begin this process:
1. With the vehicle at a complete stop, set the parking brake.
2. Put the gearshift in P (Park), turn off all accessories and start the engine.
3. Run the engine until it reaches normal operating temperature.
4. Allow the engine to idle for at least one minute.
5. Turn the A/C on and allow the engine to idle for at least one minute.
6. Release the parking brake. With your foot on the brake pedal and with the A/C on, put the vehicle in D (Drive) and allow the engine to idle for at least one minute.
7. Drive the vehicle to complete the relearning process.
• The vehicle may need to be driven 10 miles (16 km) or more to relearn the idle and fuel trim strategy.
• If you do not allow the engine to relearn its idle trim, the idle quality of your vehicle may be adversely affected until the idle trim is eventually relearned.
http://www.fordservicecontent.com/Ford_Content/catalog/owner_guides/05219og3e.pdf
Refer to your owner’s manual for the specific idle relearn procedure of your vehicle
The Ford idle relearn procedure is written in a section of battery’s maintenance and specification on an owner’s manual. It’s a good example that owner’s manuals are somewhat neglected, but actually a great source of information for your very own vehicles.
Read Also: Missing Car Owner’s Manual? You can download owner’s manual for free
Please note that this procedure is necessary for around 2000’s models of Ford vehicles. The newer models may need a different procedure from the one above. For example, just letting it idle for several minutes will have the computer relearn idle condition or the slightly different but mostly the same procedure as one above. Anyway, refer to your owner’s manual, and you will find the specific procedure for your vehicle.
The procedure is also called Battery Relearn among Ford owners in their forums, but it’s just the same thing as idle relearn, throttle relearn, resetting, calibration, etc.