Idle Speed Too Low on Vulcan Fuel Injected 3L ?
 

2001 Mercury Sable LS (4th generation, Taurus equivalent), 3L Vulcan (push-rod) engine (8th character of VIN is U)

The car starts as normally, but once started, it doesn't have the same throttle control that normally keeps the engine running at proper idle speed. If you put no pressure on the gas pedal, the engine will die. If it were an old-time carbureted engine, then I would say the idle adjust is set too low. But on a fuel injected modern engine, I don't know what controls the idle speed.

The problem is not constant. It might work ok, then have the problem, then "fix itself" for a while.

To explain further, the car can be driven normally except when you stop at a light, you need to keep a slight pressure on the gas peddle to keep the engine from dying.

What do I look at to find the problem? What do I need to test?