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Fill it right to the very top with the cap off and see if it keeps doing it. If it keeps having bubbles head gasket and or the head is not in good shape. Might have to start it a few times to get all the air that would be in there from doing the
thermostat. If that's all right might just be the rad. might need cleaning or replaced.


That isn't totally conclusive. If the cooling system was already low of coolant it may purge air for some time, those air bubbles may just be air. What would give you a better idea is one of those kits that test for exhaust gas in the radiator/cooling system.

For testing the fan clutch get an assistant. Get the truck up to operating temp. Then pop the hood and keep a look at the fan while your assistant shuts the engine off on que. It should stop very shortly after the engine is shut off. If it keeps spinning chances are that it is shot.

DougH
1997 SR - Current Lawn Ornament
1995 SR - RIP
1993 RS - RIP