Use an ohm meter to check your fuel injectors (with the engine running), by removing the wire from the injector. Two metal prongs sit inside the connector. Set the meter for the ohm setting. Touch the red meter lead to one contact and the black lead to the other prong. The ohm resistance number should be the about the same for all the injectors. If any injector reading shows a higher or lower than average number, the cause will be a defective injector, which will cause a rough idle. A zero reading will indicate a shorted injector, and must be replaced. You can also use a stethoscope or a long handled screwdriver to place on the injector head. You should be able to hear a clicking sound, telling you that the injector is "pulsing." No clicking sound indicates a dead injector.
Examine the choke setting on a vehicle equipped with a choke adjusting knob or mechanism. Make sure the choke operates by opening slowly during normal engine warm-up. A choke that fails to open or sticks shut during warm engine running will cause a constant misfire and rough idle. A choke stuck in the open position will not allow an engine to warm up properly.
Hook up a portable trouble code tool for your specific Ford make and model. Look for any trouble codes that appear while the engine runs at idle. Write down any code number and look it up in a trouble-code manual. Most times, code numbers can pinpoint the exact system failure or miss in the engine.
Powerful Autel MaxiDAS DS708 Multi language Diagnostic tool