What do you do when you completely flush out a cooling system and the car still runs hot?
Bleed your coolant
With the engine running, remove the radiator cap, turn heater on as hot as it will go (max fan speed too) and watch the bubbles come up and pop. Now add coolant untill it appears full. For some stubborn bubbles, tapping on the upper radiator hose with your thumb will dislodge them. Continue adding coolant and letting the air bubbles escape like this untill it is full and no more bubbles come out.