1997 GMC Suburban Owner's Manual - Page 311

Radiator Pressure Cap NOTICE: Your radiator cap is a 15 psi (105 kPa) pressure-type cap and must be tightly installed to prevent coolant loss and possible engine damage from overheating. Be sure the arrows the cap on line up with the overflow tube on the radiator filler neck. Thermostat Engine coolant temperature is controlled by a thermostat in the engine coolant system. The thermostat stops the flow of coolant through the radiator until the coolant reaches a preset temperature. The radiator pressure cap must be tightly installed with the arrows on the cap lined up with the overflow tube on the radiator filler neck. When you replace your thermostat, an AC@thermostat is recommended. 6-29