GMC SIERRA 1994 Manual page 292

Radiator Pressure Cap-Gas Engines
The radiator pressure
cap must be tightly
installed with the
arrows on the cap
lined up with the
overflow tube on the
radiator filler neck.
NOTICE:
Your radiator cap is a 15 psi (105 kPa) pressure-type cap and
must be tightly installed to prevent coolant loss and possible
engine damage from overheating. Be sure the arrows on the cap
line up with the overflow tube on the radiator filler neck.
Surge Tank Pressure Cap- Diesel Engines
6-42