Pontiac 2007 G6 Owner's Manual page 341

Checking Coolant
The engine coolant surge tank is located in the
rear of the engine compartment. See Engine
Compartment Overview on page 318 for more
information on location.
CAUTION:
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Turning the surge tank pressure cap when
the engine and radiator are hot can allow
steam and scalding liquids to blow out
and burn you badly. Never turn the surge
tank pressure cap — even a little — when
the engine and radiator are hot.
The vehicle must be on a level surface. When
your engine is cold, the coolant level should be at
the FULL COLD mark or slightly higher.
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