GMC 1999 Sierra 1500 Pickup Owner's Manual page 258

Prob/ems on the Road
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6. Start the engine and let it run until you can feel the upper radiator hose
getting hot. Watch out for the engine fan(s).
7. By this time, the coolant level inside the radiator filler neck may be
lower. If the level is lower, add more of the proper mix through the filler
neck until the level reaches the base of the filler neck.
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8. Then replace the pressure cap. Be sure the arrows on the pressure cap
line up like this.
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