It could be the radiator, the hoses, a leaking coolant tank or the water pump.
If it has a coolant tank, that's the "cap".
It uses the cap on the coolant recovery tank as its "radiator cap".
It could be coolant from the coolant tank or radiator, or if it's clear, it's probably just water condensation from the air conditioner.
You can find the 1991 Buick Skylark radiator drain plug on the bottom of the radiator. The radiator drain plug will be in the center of the radiator bottom.
I would say that you are losing coolant somewhere hoses,water pump,radiator.
Coolant is leaking- either onto the ground or into the engine where it mustn't.
The coolant temp sensor on a 97 Buick Century is on the side of the radiator. It is just below the radiator cap.
The thermostat on a 1991 Buick Skylark is located near the engine. To find the thermostat, follow the top radiator hose directly to the thermostat.
That is an indication that head gasket is leaking. yes!
You have to use the coolant reservoir. You'll notice it's marked as a pressure cap. If it's loose, you'll notice as your temp guage will keep going up and down instead of being steady. Many vehicles do not have a cap ON THE RADIATOR, but instead have a pressure cap on the coolant reservoir. I don't own a Buick Skylark and haven't worked on one, so I can only guess with you.
There are flush-and-fill kits that Prestone makes. It might be more efficient to have it done professionally. I think the cap used on the coolant tank acts as the radiator cap since it mentions the pounds of pressure on the label.
If there is no petcock on the bottom of the radiator then you must remove the lower radiator hose to drain it.