Hummer 2006 H3 - Manual (Page 304)

How to Add Coolant to the Radiator
1. You can remove the radiator pressure cap when
the cooling system, including the upper radiator
hose, is no longer hot. Turn the pressure cap
slowly counterclockwise about one full turn.
If you hear a hiss, wait for that to stop.
A hiss means there is still some pressure left.
2. Then keep turning the cap to remove it.
3. Fill the radiator with the proper DEX-COOL ®
coolant mixture, up to the base of the filler
neck. See Engine Coolant on page 5-24 for more
information about the proper coolant mixture.
4. Then fill the coolant recovery tank to the
FULL COLD mark.
5. Reinstall the cap on the coolant recovery tank,
but leave the radiator pressure cap off.
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