There is a technical service bulletin related to this problem, I am sending you. If there is air in the system you will need to bleed the air from the system. I would recommend to change the thermostat, do the service bulletin procedure and install a new cap. Then bleed the system of any air. There is an air bleed made of brass usually sits on or near the upper radiator hose inlet on the intake.
Bulletin No.: 00-06-02-001
File In Section: 06 Engine/Propulsion System
Date: January, 2000
Engine Running Hot, Overheating and/or Loss of Coolant (Polish radiator Filler Neck and Replace radiator cap. Models:
1999-2000 Passenger Cars and Trucks with Composite radiator End Tank
Some customers may comment on one or more of the following conditions:
|^||Engine running hot |
|^||Engine overheating, and/or |
|^||Loss of coolant/low coolant message |
The radiator filler neck may have an imperfection in the sealing surface.
Important: DO NOT REPLACE THE RADIATOR.
Using a piece of 400 grit wet/dry sandpaper backed with a flat piece of wood, polish the filler neck sealing surface using a circular motion.
Replace the radiator pressure cap with a cap of the same part number as shown in the GM Service Parts Catalog.