2005 Pontiac Bonneville Owner's Manual - Page 295

If you have to add coolant more than four times a year, have your dealer check your cooling system. Notice: If you use the proper coolant, you do not have to add extra inhibitors or additives which claim to improve the system. These can be harmful. Checking Coolant The vehicle must be on a level surface. When your engine is cold, the coolant level should be at the FULL COLD mark which is located on the side of the surge tank that faces the engine. If the LOW ENGINE COOLANT message comes on and stays on, it means you are low on engine coolant. See DIC Warnings and Messages on page 3-62 for more information. 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. {CAUTION: Turning the surge tank pressure cap when 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 engine coolant surge tank is located toward the rear of the engine compartment on the passenger's side of the vehicle. See Engine Compartment Overview on page 5-12 for more information on location. 5-35