Couple possible things.
I had a bad battery that wouldn't hold enough of a charge to run the engine when you stopped (because the engine wouldn't be running fast enough to charge it).
Or, idle air control valve.
If this dodge van is an Xplorer You have a larger than normal coolant capacity as the coolant is used to heat water in your water heater for more. Xplorer Owners Club at Yahoo Groups .com
The engine computer is the voltage regulator on a 1991 Dodge.