1994 GMC Sonoma Owner's Manual - Page 196

3 The coolantlevel should be at or above the ADD mark. If it isn't, you may have a leak in the radiator hoses, heater hoses, radiator, water pump or somewhere else in the cooling system. NOTICE: Engine damage from running your engine without coolant isn't covered by your warranty. If there seems to be no leak, start the engine again. See if the fan speed increases when idle speed is doubled by pushing the accelerator pedal down. If it doesn't,your vehicle needs service. Turn off the engine.