Loosing antifreeze but no visible leaks and none in engine oil. Also heater blows cold. Could thermostat be causing it to blow it out of overflow tube?

If your 1998 Cutlass is overheating and the heater is blowing out cold air, and you are also noticing that coolant is leaking out, there are a few possible causes. Here are some things to check:

Check for a coolant leak: Start by checking the engine and radiator for any visible signs of a coolant leak. It's possible that a hose or other part of the cooling system may be damaged or loose, causing coolant to leak out.

Check the radiator: If the radiator is not working properly, it can cause the engine to overheat. Check to make sure the radiator is not clogged or damaged.

Check the water pump: The water pump circulates the coolant through the engine. If it's not working properly, coolant may not be flowing through the system correctly, causing the engine to overheat.

Check the cooling fan: The cooling fan helps to keep the engine cool by pulling air through the radiator. If it's not working properly, the engine may overheat.

Check the thermostat: Even though you recently replaced the thermostat, it's possible that it's not working correctly. A faulty thermostat can cause the engine to overheat.

It's important to address any issues with the cooling system as soon as possible, as overheating can cause serious damage to the engine. If you are not comfortable diagnosing and repairing the issue yourself, it's recommended to take your car to a qualified mechanic.

Some things it could be but not specific to your vehicle.
Do you smell antifreeze? Engine temps running hot? Has it run hot?
No antifreeze in radiator, when cold take radiator cap off and look to see if it is full. add as needed in radiator and overflow container.
Heater control valve not working properly or inop.
Clogged heater core. Leaking heater core-heater core bypassed.
Heater box door not operating properly or inop.
Vacuum leaks or broken lines/cables.
Intake manifold leaks
Clogged radiator
Could be many things, checks/tests need to be performed to determine the cause either by you or a certified repair shop.

If your thermostat is staying open then it won't get hot in the cab,if the heater is bad,it will leak and fog up the window,another cause could be the blend door is stuck close,or the valve is stuck close and antifreeze can;t circulate thru the heater core.When you first start your car does the thermometer on dash go up then cone down,if it is then thermostat is good.

it sounds like your thermostat is stuck shut..and the antifreeze is getting blown out during operating as you drive it..when replacing this thermostat I have a trick for you to do with it..since the thermostat is basically a mechanical open/close valve..sometimes it screws up and sticks shut..this causes the antifreeze to overheat in the head and blow out of the top tank/hose area on the car(there is an overflow hose there)on the radiator..now when you go to install the new thermostat.. try drilling 2 small holes at a 90* to the bridge over the actual spring-loaded valve.. these go into the wide flange that surrounds the sides of the thermostat, as close to the center as you can go without going thru the center.. these will be called weepholes..IF there are any hairline cracks in the head(s) these weepholes will allow the air to go thru without dropping the antifreeze levels in the heads..the pressure will bleed off out of the radiator cap.. and your heater will still stay warm..

Sounds like the problem lies in the thermostat, When the engine thermostat fails to close fully, the engine coolant temperature may fall. Low engine temperature causes several problems and may cause the heater to blow cool air. Make sure about that.

Heater coil

glad you found the problem

OVERHEATING CAN BE CAUSE BY MANY THINGS LIKE FAULTY THERMOSTAT AND FAULTY RADIATOR PRESSURE CAP,TOO MUCH WATER IN COOLANT SYSTEM NEED 50 / 50 WATER AND ANTIFREEZE.LOOK FOR WATER PUMP WEEP HOLE LEAK, IF COOLANT LEAKING OUT WEEP HOLE WATER PUMP BAD NEED REPLACING.CHECK FOR LOW COOLANT LEVEL, YOUR RADIATOR OVERFLOW JUG SHOULD HAVE COOLANT AT THE FULL COLD MARK IF NOT KEEP ADDING COOLANT INTO COOLANT OVERFLOW JUG UNTIL COOLANT STOP DROPPING AND FULL COLD MARK, IF OVERFLOW COOLANT JUG WAS EMPTY RADIATOR IS LOW ON COOLANT.WHICH CAUSE AIR IN COOLANT SYSTEM KEEPING THE ENGINE COOLANT TEMPERATURE SENSOR FROM BEING SUBMERGE IN HOT COOLANT WHICH TURN ON THE COOLANT FANS.WHEN ENGINE COOLANT TEMPERATURE GET CERTAIN SET TEMPERATURE LIKE 190 DEGREES THE PCM AND COOLANT SENSOR WILL TURN ON COOLANT FANS, IF VECHICLE OVERHEATING COOLANT FANS NOT WORKING EITHER YOU HAVE FAULTY ENGINE COOLANT TEMPERATURE SENSOR CHECK COOLANT FANS FUSE AND RELAYS. YOU CAN CODE SCAN CAR TO SEE IF YOU HAVE FAULTY PCM, FAULTY ENGINE COOLANT TEMPERATURE SENSOR OR FAULTY COOLANT FAN RELAY.IF COOLANT FAN AND ENGINE COOLANT TEMPERATURE SENSOR OKAY,YOU HAVE CORRECT COOLANT LEVEL IN COOLANT SYSTEM, THERMOSTAT AND RADIATOR PRESSURE CAP BEEN REPLACED, YOU COULD HAVE BLOWN HEAD GASKET CHECK ENGINE OIL LOOKS LIKE MILK SHAKE HEAD GASKET BLOWED ALSO WHEN HEAD GASKET BLOWED YOU CAN DRIVE CAR SHORT DISTANCE START OVERHEATING AND YOU LOOSE ENGINE POWER SUSPECT BLOWED HEAD GASKET.

If there is a internal antifreeze leak, there will be antifreeze mixed in your oil. Check your oil. If there is antifreeze in there, you probably have a bad head or base gasket. Worst case scenario is your block is cracked somewhere. Check your heater core and the lines as well. If the heater core is bad it will be leaking.
Check the thermostat housing for a leak. Sometimes
the antifreeze will leak from there, and burn on top of the exhaust instead of dripping onto the ground. Good luck

That wouldn't be fuses. You probably have a bad thermostat (causing the engine to not reach proper operating temperature) low antifreeze, a bad blend door in the climate control system, or a blocked heater core. Based on the age of your vehicle, I'd say thermostat first, then low antifreeze for likelihood.