Save your money on the Prestone.
There are 2 block drain plugs, only one of which is easily accessible. It is on the left hand side of the block, above the starter. It is a 3/8" pipe plug, which has a 9/16" head. Make sure you get that out, and make sure you flush the heater core. I access the heater core at the connection just above the water pump.
Lots of flushing, lot of water. You may have to do it again in the fall.
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Bob
2002 Cobalt 226 350 MPI B1