1998 GMC Sonoma Owner's Manual - Page 278

Radiator Pressure Cap Thermostat Engine coolant temperature 1s controlled by a thermostat i n the engine coolant system. The thermostat stops the flow of coolant through the radiator until the coolant reaches a preset temperature. Power Steering Fluid I NOTICE: Your radiator capis a 15 psi (105 kPa) pressure-type cap and must be tightly installed to prevent coolant loss and possible engine damage on from overheating. Be sure the arrows the cap line up with the overflow tube the radiator on filler neck.