Pontiac 1993 Sunbird Owner's Manual page 134

How to Add Coolant to the 2.0L
L4
Engine at the Thermostat Housing
1.
You can remove the thermostat
housing pressure cap when the
cooling system, including the
thermostat housing pressure cap and
upper radiator hose, is no longer hot.
Turn the pressure cap slowly to the
left 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 slill
some pressure left.
2.
Then keep turning the pressure cap,
but now push down as you turn it.
Remove the pressure cap.