Locate the drain plug on the bottom of the radiator. Find the faulty thermostat. It is located directly under the power steering reservoir and has three hoses running from it. Use the screwdriver to remove the hose clamp on the hose entering the engine. Pull the hose off and use the socket set to remove the cover underneath.
Pull the thermostat out and replace it with a (160 or 180) Stant thermostat. I'm not recommending either or on the temp but a lot go with the 160. Replace the cover, reinstall the coolant hose, and refill the radiator with a 50/50 mixture of water and coolant. Start the car and allow it to heat to 210 degrees F. Observe the in-dash temperature gauges to pinpoint any inconsistencies in temperature.
Have your coolant crossover tube loosened and off so that when the car heats up, the system will allow itself to burp out any air left in the cooling system. If air is left, it's likely that as you drive down the HWY, the temp will rise and in your mind, you'll be thinking it's full of water and it will be but air will also be in there causing the temp to rise. Make sure to burp the system of all air, screw cap back on crossover tube and tighten. Take it for a test drive. If you see the temp gauge rise above the 1/2 way mark, you most likely have air still in the system.
Let it cool before trying to remove the cap again. That's the highest point of the systems water flow and that's why you want to fill there. Also, fill up the reservoir but don't think just because it's showing full that the entire cooling system is full. Must burp it!