2008 Cadillac CTS Owner's Manual - Page 311

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 FROID/FULL COLD 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 5-14 for more information on location. 5-29