Pontiac 1998 Trans Sport Owner's Manual page 419

Short Trip/City Maintenance Schedule
150,000 Miles (240 000 km)
I
DATE
0
Drain, flush and refill cooling system
(or
every 60 months since last service,
whichever occurs first). See "Engine Coolant" in the Index for what to use.
Inspect hoses. Clean radiator, condenser, pressure cap and neck. Pressure test
cooling system and pressure cap.
An Emission Control Service.
7-20