These v-8's are difficult to purge correctly. Have them replace the thermostat again, just to be sure they have a good part in the car. Now there is a tool called an Airlift. This thing will vacuum all of the air out; then suck coolant into the void. This will get all of the air out of the engine.
Be sure the auxillary fans are working.
1991 GMC Syclone
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