The rad fill cap is not on the radiator. It is located on the pass side of the vehicle by the front pass headlight. Usually has a thermometer looking thing on the cap. Just make sure your engine is cool before removing a radiator cap, if not you can burn yourself pretty bad.
On a 1998 Ford Expedition : The thread on cap on your engine coolant reservoir is the pressure cap / radiator cap
The radiator cap is on top of the radiator.
On a 2002 Ford Expedition : The thread on cap on the engine coolant reservoir is the pressure cap / radiator cap
On a 1998 Ford Expedition : The thread on cap on your engine coolant reservoir is the 16 PSI pressure cap / radiator cap
On a 2001 Ford Expedition : The thread on cap on your engine coolant reservoir is the ( 16 PSI ) pressue cap / radiator cap
On a 1998 Ford Expedition : The thread on cap on your engine coolant reservoir is the 16 PSI pressure cap / radiator cap
The thread on cap on your engine coolant reservoir is the " radiator " cap
On a 2001 Ford Expedition : The thread on cap on your engine coolant reservoir is the pressure cap / radiator cap
The thread on cap on your engine coolant reservoir is the " radiator cap "
On your 1999 Ford Expedition : The thread on cap on your engine coolant reservoir ( located at the front of the engine compartment , on the drivers side ) is the pressure cap / radiator cap
On a 1997 Ford Taurus the thread on cap on the engine coolant reservoir is the pressure cap / radiator cap
On a 1997 Ford Taurus : The thread on cap on the engine coolant reservoir is the pressure cap / radiator cap