The idle air control motor will set a code on that vehicle. Seeing that
it is a 1998 it would be OBDII. However just because it stays at 1500
rpm during idle doesn't make it the IAC motor.
You need to check for vacuum leaks. Listen for any hissing type sounds
in the engine area.
The IAC motor essentially creates a controlled vacuum leak to make the
idle go up or down.
If you have another leak then it can't control itself. I can't say this
is the best way, but I like to use carb ceaner (use the supplied straw)
and spray the intake area with small squirts and see iff the idle picks
up. if it does then look in that area for leaks if you can't hear them
already. ******doing this can cause a fire though on a hot engine. Be
careful if you do this.******