I go about this the same way on every car.
Step 1 is to replace the thermostat. Now drive the car and come back and turn off the engine. Immediately open the hood and hold the fan blade in a position so you can get one hand on the radiator. If the radiator is HOT, the auxillary fan and or the fan clutch on the main fan is bad.
Be sure to check the auxillary fan fuse...
1991 GMC Syclone
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