Ok, the thermostat allows the engine to get to operating temp before allowing water to flow through the cooling system.
The fan is controlled by a small temp switch normaly somewhere in the top radiator hose, or the top of the thermostat houseing. (or similar place near the engine out let end of the coolant flow)
An air bubble can form almost anywhere in an engine and for a variety of reasons, but unless you have a coolant leak somewhere, or have drained and refilled the system with out correctly bleeding it, then its VERY unlikely its the problem.
Being a 93, I would assume it still is. But check the fuse's, and if you have one check the fan is getting power with a multimeter or test light. (remember to do with when the engine is at operating temp). If its getting power but not turning on, the fan is the problem, if its not getting power then it could be a fuse, faulting wiring, or faulty thermo switch.
Or, it could be the thermostat has stop jammed, or simply failed, and isnt opening when the engine warms up. Very common problem, especialy if you have never had the coolant changed.
The easy fix is to simply replace it.
