Jay Giron 1/7/2012You might have air trapped in your cooling system.
Things You'll Need
Add a mixture of one-half water and one-half antifreeze to the radiator. Fill it right up to the top.
Fill the overflow/coolant reservoir with the same 50/50 mixture.
Leave the radiator cap off, turn the engine on and let it run until the radiator "burps": You will see the coolant level drop and may see or hear a large air bubble come to the top as the system burps.
Keep an eye on the temperature gauge throughout this process.
Refill the radiator to the top and coolant reservoir as needed.
Put the radiator cap back on.
Note that if the engine runs hot after this procedure there may have been another pocket of air that "burped." Let the engine cool down and then add more coolant to both the radiator and the coolant reservoir.