Ford Repair: Expedition idle problem, rubber elbow, air control valve

Question
I have a 1999 exepadition it has an intermitting problem. In the past year it has started to idle after it is hot around the 400 rpm range. When it is cold it will just die. The problem just starts and stops at random I might go two weeks or it might be just a day. When it is running correctly  it maintaines the 700 rpm. I have not had to do anything other then oil and filter changes.  I took it to the local parts store they hooked up a dianostic conputer to it and it had no troble codes. Can you give me some direction I'am very mechanical inclined, I just need direction to start looking.  

Answer
There are two common problems on Expeditions that can cause this symptom. One is a leak in the pcv hose, usually at the rubber elbow at the intake manifold, this is more common on the 5.4L but you didn't say which engine you have. The other is the idle air control valve, which can stick and not control idle speed. With the absence of codes I would suspect one of these is your problem.