install the thermostat valve back in the engine - and while doing that check the condition of the waterpump metal inlet pipe with a mirror and light. It should be clean and not packed full of rust. If it is the coolant will never reach the radiator fully and just circle around in the engine.
To add coolant - remove all of the old coolant/water and use 50% or at least 30% conc coolant in your engine. for 50% mix equal quantity of coolant with water for 30% mix 1 part to 2 parts water, prepare in bucket pour in system and remove air pockets from bleeder nipple - it will be in the front of the engine (if equipped) if not - park the car on a slope facing upwards and fill - while taking out any air pockets.