Pontiac 2004 GTO - Manual (Page 225)

3. Then fill the coolant surge tank with the proper
mixture to the COLD FILL mark.
4. With the coolant surge tank pressure cap off, start
the engine and let it run until you can feel the upper
radiator hose getting hot. Watch out for the
engine cooling fan.
By this time, the coolant level inside the coolant
surge tank may be lower. If the level is lower than
the COLD FILL mark, add more of the proper
mixture to the coolant surge tank until the level
reaches the mark.
5. Then replace the pressure cap. Be sure the
pressure cap is hand-tight and fully seated.
See your dealer, if necessary.
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