1996 GMC Sonoma Owner's Manual - Page 250

Radiator Pressure Cap NOTICE: Your radiator cap is a15 psi (105 kPa) pressure-type cap and must be tightly installed to prevent coolant loss and possible engine damage from overheating.Be sure the arrowson the cap 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 tlow of coolant through the radiator until the coolant reaches a preset temperature. 6-28