Pontiac 2002 Bonneville - Manual (Page 279)

5-16
How to Add Coolant to the Radiator
1. You can remove the radiator pressure cap when
the cooling system, including the radiator pressure
cap and upper radiator hose, is no longer hot. Turn
the pressure cap slowly counterclockwise until it
first stops. (Don’t press down while turning the
pressure cap.)
If you hear a hiss, wait for that to stop. A hiss means
there is still some pressure left.
2. Then keep turning the pressure cap, but now push
down as you turn it. Remove the pressure cap.
3. Remove the 3800 Series II V6 engine cover shield to
access the bleed valve.