low coolant but no leak

Had same problem with a 2001 Mercury Sable the way I stopped the leak is removed top hose going to radiator and poured some Aluma Seal Stop Leak into hose then reconnected hose and that took care of the could not found anti freeze leak.

Only will clog if you use more then what is recommended if does not stop leak the first time do not continue to add more hoping it will work that is when you can clog up the cooling system.

Cars with coolant overflow bottle, but with no radiator cap: These are problematic in getting the air bled out of the cooling system. This causes overheating, over-pressure leaks, hard on the head gasket, etc, I've been there, especially with my 2001 Grand Am. New water pump, thermostat, coolant overflow bottle; and it still spit water out of the weep hole on the water pump and thermostat gasket. No change until I got all of the air out of the cooling system. To Bleed Air: Run motor until hot, open the coolant overflow cover, fill the coolant overflow bottle fulll to the top, keep it full until big bubbles come. This WILL take out the air. The bleed screw/s on the motor can help if you have two bleed screws. If you have only one bleed screw on the motor, with the second bleed screw place having a hose going back to the overflow bottle, it is much better set up. When the bubbles come at the overflow bottle open cover, you will also see the small hose bubbling also. Keep it up for a little while (that's all it takes), and keep adding coolant until near the top. When the engine is cool, the coolant should contract back to the "cold coolant level' marked on the side of the coolant overflow bottle. Subtract some coolant at the bottle, if necessary when cold. When I got all of the air out of the engine, I had no more leaks at the water pump and no more leaks at the thermostat area. It now all works as it should.

I have a 99 Pontiac grand am and I just changed out my old thermostat and put a new on in there so why does it keep spitting coolant out in the center of my engine on to the ground it’s piles of coolant on the ground what else can it be

3.5 manifold gaskets are known to fail. Sometimes when they do the coolant is lost in the exhaust. So not always a head gasket issue. Can be a manifold gasket. Bleeding the air from the 3.5 is critical