2003 GMC Sierra 1500 Pickup Owner's Manual - Page 410

Adding Coolant If you need more coolant, add the proper DEX-COOL@ coolant mixture at the surge tank, but only when the engine is cool. Turning the surge tank pressure cap l...en the engine and radiator are hot can allow steam and scalding liquids to blow out and burn you badly. Never turn the surge tank pressure cap - even a little- when the engine and radiator are hot. The vehicle must be on a level surface. When your engine is cold, the coolant level should be at the FULL COLD mark. If the LOW COOLANT LEVEL message comes on and stays on, it means you're low on engine coolant. You can be burned if you spill coolant on hot engine parts. Coolant contains ethylene glycol, and it will burn if the engine parts are hot enough. Don't spill coolant on a hot engine. I When replacing the pressure cap, make sure it is hand-tight. See "Low Coolant Level" under DIC Warnings and Messages on page 3-60. 5-35