would useing 100% water for coolant cause overheating?

what coolant does is raises the temperature at which water boils. When your water begins to boil, the air bubbles will prevent the water from making full contact with the inside of your engine, therefore it will not cool effectively. Also, coolant helps prevent rust and deposits in your engine. Running without a thermostat will cause it to take longer to reach normal operating temperature, resulting in lack of HP and Mileage. By the way, too much coolant is bad as well because it will not cool as fast as water when it passes through the radiator. Use a 50/50 pre mix if you don't want to mix it yourself. never use more than 50% water and never less than 25% water. Put the proper thermostat in. If your engine runs hot, something else is wrong. Flush the cooling system thoroughly and replace your radiator cap. If it doesn't hold the correct pressure, it will cause overheating. Make sure you have sufficient flow and your water pump is not bad, usually indicated by a very small leak at what's called a wheap hole at the bottom of your water pump. Sometimes they can be bad and not leak though.

There may have been air pockets in the engine block after refilling with water. Running dry and overheating risks serious engine damage, particularly to the cylinder head from warpage or cracking, and head gasket failure.

Water loss will cause overheating. Straight water boils in the pressurized system at probably 230 degrees. If that's wrong, I'd love to be corrected. When enough pressure builds up it will vent out of the radiator cap. Steam pockets in the block are hot spots where heat cannot be removed by coolant, aggravating the situation. The boiling point of 50/50 antifreeze and water is much higher so the coolant is more resistant to boiling and coolant loss. If you took out the thermostat, the engine could not run at it proper operating temperature and would waste fuel.

You can do it to test the car, but you will want to flush it after diagnosing and then refill properly. No need to waste a bunch of coolant on a test if you're going to drain it out right afterward. The coolant protects the engine and radiator from being ruined by straight water, but you should be able to run water through it for a short period of time without causing any damage.

A 50/50 mix adds approx 40 deg to the boiling point.

Low coolant level is a first choice.

A defective stat is another.

A contaminated system is also.

Oh hell yes, please don't do it!!

Adding a thermostat will do nothing, use coolant and don't cut corners here, purely water is a definate no no unless you want to overheat and seize your engine solid.