Ford Repair: engine cold start, upper intake manifold, air control valve

Question
1998 Ford Explorer 4.0 SOHC
    
    After setting for about 8 hours, and temp. drops below 50 degrees - engine starts - goes to high idle - comes down slowly to where normal idle speed would be and continues to drop - engine stumbles - sometimes it goes back to high idle and repeats the sequence - if too cold, it dies. Once the engine warms up a little it idles and runs perfactly. After setting for only a fiew hours, as long as the engine has not gotten to cold it starts and runs normally.  

Answer
If the Check Engine Light is NOT on, I would guess that the Idle Air Control Valve (IAC) is bad.  Bad IAC's are common on Explorers, but they usually cause a hard start, or stalling condition.  When starting the engine and at idle, the throttle plate is closed and the only air entering the engine is through the IAC. This is all controlled by the Powertrain Contol Module. It uses the IAC to allow a small amount of air to enter the engine for starting and to adjust idle speed. The IAC is mounted to the upper intake manifold with two small bolts, and has a small electrical connector on it. It is about two by four inches. It may be sticking when cold, you can remove it (be careful, it may have a paper gasket) and clean it out with an Air Intake Cleaner. If this doesn't help, it may have failed and need replacing.
The engine could also have a vacuum leak at the o-rings between the upper and lower intake manifolds, but this would normally turn the Check Engine Light on.

I hope this helps,
Dave