Apparently when you purge the coolant system, GM wants you to add two stop-leak pellets to the system -- through the t-stat, so it doesn't plug the reservoir tank. You can buy them at the parts counter. Sounds kind of hokey to me, but I'm told it's in the service manual.

 

LOL! Okay! The last time I added stop leak pellets to a GM radiator (10 y/o 83 Chevy Cavalier), it was the middle February, and it plugged my heater core up in about 45 seconds in a middle of a snowstorm. Later that afternoon, I was the only customer in a large south Jersey Ford dealership. I remember the guy looked at us two, blue from the cold and said rubbing his hands, "you folks must really need a new car!" It took about 5 minutes to pick out a new '93 Escort for the wife (which my niece is still driving today!).

 

I lied, I don't have a radiator cap... I apologize bout that.

 

If you have to add stop leak, only use Bar's. It will not stop up a heater unless the heater is already partially plugged.