two most likely things to cause a surging or rough idle are
1. vacuum leak
2. dirty sticky IAC
What does the IAC do? How does it work?
http://www.fordfuelinjection.com/?p=39
Check and clean your IAC
http://www.ford-trucks.com/article/idx/8/1...er_IAC_Fix.html
TheOldWizard Has some issues with the video I'll link to below, and while I agree with him, I think that overall the video is helpful and worth looking at.
First, the example Idle Air Control/Bypass (IAC/IAB) motor they showed during most of the video is very different from the style used on most (all?) Ford vehicles. They showed a Ford design briefly and described it as one with an internal pintle.
Second, they said specifically not to spray cleaner on IAC with internal pintles. BS ! Any carb cleaner will work. Most will dry quickly. Just make certain to not leave any gunk.
Third, I have seen IACs work properly when warm, but stick when cold so their suggested testing procedures may not be perfect. In other words, you could pass all of the tests and still have a sticking IAC when cold.
youtube video
http://www.youtube.com/watch?v=406BTo4JE3A...player_embedded
Tips for Checking for vacuum leaks.
Some of the sensors and some of the actuators either read the amount of vacuum, or use vacuum to move controls, therefore it is important to the proper functioning of the engine that your vacuum system is fully intact. Otherwise one may get codes falsely indicating that there is a problem with a controller/actuator, or a sensor when in reality it is caused by a lack of vacuum.
Look at your rubber vacuum hoses (they are about 1/4 - 3/8 inches in diameter, if they are dry rotted or cracked, replace them. IF they slip on/off of their connectors too easily, they may have stretched out a little bit, cut the ends off, and put them back on.
You may listen for a leak. Sometimes using a cut off piece of garden hose is a good aid to listen with. Sometimes a cardboard tube (like from gift wrapping paper) is helpful. Just remember that whenever you stick your head inside a running engine compartment, that you are not wearing any loose clothing or jewelry that can get caught.
Another suggestion is to use a spray can of carb cleaner and spray around the bottom of the intake manifold, and the base of the throttle body assembly. IF there is a leak, you should hear a change in your engine.
Keep a fire extinguisher handy, or use water instead of carb cleaner (it works, but not as well.) Be aware that if you use water and the wires get wet and it runs worse... your wires may be bad, and the water is allowing them to arc instead of jump the gap of the spark plug.
Another suggestion is: to take a propane torch, turn it on (but don't light it) and move it over the vacuum hoses and connections. The engine RPM's will increase if there is a leak and the propane gets sucked in.
Courtesy of TheOldWizard ford-forums... Don't forget to remind folks to check the PCV valve, hose and grommet !
Courtesy Ramnasal ford-forums... Clamp off the hose that runs to the vacuum booster (brake booster) or pull it and plug it. It is possible that the diaphram on the inside of the booster may go bad and cause a vacuum leak.
And one more place that is not always obvious is that the vacuum reserve canister in older vehicles sometimes rots out and leaks, but rarely gets checked. In older vehicles it looks like a metal juice can, in newer vehicles it is made of plastic.
