2007 Cadillac STS Owner's Manual - Page 391

Checking Coolant {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 vehicle must be on a level surface. When your engine is cold, the coolant level should be at the FULL COLD/FROID line on the side of the surge tank. Follow the arrow from the top of the tank down the side to the horizontal mark. The coolant surge tank and pressure cap are located on the driver's side of the vehicle, toward the rear of the engine compartment. See Engine Compartment Overview on page 373 for more information on location. 391