99 olds cutlass, this morning I drove it and the coolant low light came on. Stopped at autozone, grabbed some coolant and tossed some in. Kept driving, light was intermittently on and off. Drove it to work and around noon filled it pretty full with coolant. Drove away at 5pm, no puddle underneath the car. Stopped to get gas, didn’t notice any puddle but didn’t look closely. Drove it home, parked it, went to get the mail, came back, pretty impressive puddle of coolant leaking from under the car.
If you overfill the coolant reservior and then drive the vehicle, coolant can be purged from the cooling system after the engine is turned off because there’s no room for expansion, so the coolant comes out of the pressure cap.
As to why you’re loosing coolant can be determined by performing a coolant pressure test. In the meantime, you might want to pull the oil dipstick out and check if the oil is contaminated with coolant.
Coolant contaminated oil will take on a milkshake-like appearance. You could also drain the oil and look closely at it as it comes out. The coolant will come out first. A pressure test will point to where the leak is coming from, and if you have the 3.1L V6, the odds are against you for a leaking lower intake gasket. They are very common, and can indeed leak into the crankcase, destroying the engine if not fixed promptly.