There is a Technical Service Bulletin for your situation.
ENGINE RUNNING HOT/OVERHEATING &/OR LOSS OF COOLANT
TECHNICAL SERVICE BULLETIN
Reference Number(s): 00-06-02-001, Date of Issue: January, 2000
Related Ref Number(s): 00-06-02-001
ENGINE RUNNING HOT, OVERHEATING AND/OR LOSS OF
COOLANT (POLISH RADIATOR FILLER NECK AND REPLACE
Model(s): 1999-2000 GM Passenger Cars and Trucks with Composite Radiator End Tank
Section: 06 - Engine/Propulsion System
Bulletin No: 00-06-02-001
Date: January, 2000
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.
NOTE: 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
Monday, January 12th, 2009 AT 9:58 PM