I have an 88 gmc with the 5.7 and it says it's overheating after driving it for only a few minutes and I can hear it bubbling/boiling in the engine so I replaced the thermostat but it still says it's overheating. Could it be air bound? Or something else?
It could be air bound or it could be another issue causing air to be injected into the engine cooling system.
I would first try to get the air out by raising the front of the truck using ramps or jack stands and run the engine with the radiator cap off. Continue filling the radiator with the engine running until it does not take anymore. Then test drive the vehicle. This could take awhile because the thermostat has to open and close letting out the air when the thermostat is open.
The problem could be just being air bound or it could be a head gasket or cracked head.
Get back to me when you can to let me know how you make out. We can diagnosis the problem further if needed.
Also, a quick check for a blown head gasket is to take a whiff inside the the radiator or expansion tank. If you smell exhaust (combustion) it's a done deal.