2008 GMC Sierra 1500 Pickup Owner's Manual - Page 343

How to Add Coolant to the Coolant Surge Tank If you have not found a problem yet, check to see if coolant is visible in the surge tank. If coolant is visible but the coolant level is not at or above the FULL COLD mark, add a 50/50 mixture of clean, drinkable water and DEX-COOL® coolant at the coolant surge tank, but be sure the cooling system, including the coolant surge tank pressure cap, is cool before you do it. See Engine Coolant on page 5-23 for more information. If no coolant is visible in the surge tank, add coolant as follows: {CAUTION: Steam and scalding liquids from a hot cooling system can blow out and burn you badly. They are under pressure, and if you turn the radiator pressure cap - even a little - they can come out at high speed. Never turn the cap when the cooling system, including the radiator pressure cap, is hot. Wait for the cooling system and radiator pressure cap to cool if you ever have to turn the pressure cap. 5-31