Coolant level is going down without any obvious leakage
Maybe you have a blown head gasket. Check the oil level and color. Is the level too high? Does it look like chocolate milkshake? - Blown head gasket or cracked head or cracked block and water in your oil
Maybe your intake manifold gaskets have failed or the manifold itself has failed, which, if I rmember correctly can be a problem on these cars.
As a follow-on, look on the inside of the oil cap. If it looks like a chocolate milkshake, you have a blown head gasket.
It’s most likely the lower intake manifold. 2 Buick LeSabre’s, 2 manifold gaskets.Not a DIY for sure.
The new gasket will most likely solve the problem. Not cheap, but get it done before you wreck the engine.
Any GM dealer can tell you about it.
Coolant entering the cylinders via the head gasket or intake gasket can damage the cat also, another reason to seek immediate pro help for this. And it still might be something simple, like the water pump is leaking a little, and the coolant evaporates away as it lands on the hot engine parts.
I’ve got several vehicles with this same engine. I’ve never had a problem with intake manifold gaskets, nor have I ever had head gasket problems.
I wouldn’t rush to to replace either of those.
In my experience and my humble opinion…
…If it’s not the water pump (and those are most often visible leaks) then it’s most likely to be the 2 little black plastic heater by-pass elbows that run between the serpentine belt tensioner body and the heads.
They are made from plastic that ages and through the heating process becomes very brittle and develops cracks. Sometimes the cracks are fine enough to lose coolant, but not so much as to be evident. They are mostly hidden from easy view.
A pair of aluminum replacement elbows (with seals) runs about 15 bucks and would probably take a novice an hour or two to install, not a major job. The hardest part is picking out pieces of crumbling plastic.
With a light and tiny mirror you can poke around and possibly spot some wetness in that area if they’re leaking. Try stuffing a tissue back there and pulling it out to see if it comes out with wet coolant stains.