99 jeep grand cherokee won't idle. just happened. ideas why?

Agree with C-Tech.

We've had two Jeeps (both with 4.0L I-6 engine). Had similar symptoms. Both times it was the idle control stepper motor. Both times they failed without warning. Both times, the only way I could get the vehicle to start would be to put my foot on the gas pedal when cranking.

As of 2006, the cost on the part was approx US$80 to US$100. Easy to replace. Held in place with two screws. This is a relatively common failure item on Jeeps (at least that is the impression I got from the parts & service dept when I explained the symptoms). This was a repair I was able to complete in under 30 min using the shop manual.

Jeep Cherokees and Grand Cherokees had problems with a defective throttle place sensor (TPS). replace is the only "fix" however the area is available at NAPA particularly inexpensively.

You may have a dirty throttle body and a stuck IAC (Idle Air Control) motor. Clean the throttle body, if it will idle ok, you're fine. If it still has a problem, replace the IAC motor. Good Luck