I am revising this cause at the last time I was pretty frustrated... Again 4.9L 6 went to start it up this morning and noticed an idle fluctuation of 500 rpm would go from 500rpm to 1000 not a miss noise either. It was running rich though. I checked for vacum leaks none found. I checked the lines from the smog pump and fuel system thermactor. None found I noticed the other day with the belt off when I turned the smog pump pulley a squeaking noise still its a pump it mooves air. I dont think this is the problem. By the time I reached work it was still indicating on the poor temp hand gauge as if it was cold. Before leaving I ran it hard down the big highway and back. Checked the upper hose warm, checked the temp of the coolant in the block warm, heat very hot. To me the aftermarket thermostat I put in was working. It did not seem to be running in open circut and obviously not closed or it would run hot. Checked the resistance to the temp sensor seemed okay dont have the exact specs. Checked temp sensor corroded and old. Decided to replace ECT in a short distance to go to the ford dealer for the part it shot some hard code (check engine light). It has not come on since. I replaced the ECT did not clear the codes. Truck still ran bad. If I knew which wires to jump I would retrive the code or codes. To me its probbably a running rich code, but its hard to say. So with the cooling system malfunctions being outruled, it leads me to my next question what is it? I have suspected a potential stuck EGR Valve, the solenoid maybe not sure how that works on a Ford. Maybe a bad ignition modulator, doubt a IAC usualy then you have an excessive idle as if the vehicle could accelerate itself. Any knowledgable answers are appreciated. I forgot to mention that now it will not run at idle unless you keep your foot on the throttle. It feels as if the engine and transmission could drop. Also the current tempature today is 20 degree mark. I know fords hate the cold weather but I would not think to the severity that mine is. Most bizzare is how it was colder last night and this problem did not occur. It did have trouble starting, but it would run smooth at idle. Now I have a hard time starting it, and its impossible to keep running at idle. Thanx

 

Not idling without giving it throttle could be a bad or very dirt IAC valve. Try cleaning it. It could also be the throttle position sensor. You can test the voltages....I'm not sure what the exact readings should be, but http://fordfuelinjection.com can help.