blocked radiator either restricted in side from corrosion or cardboard in front of the fins between the condenser, could be a bad fan clutch but it should cool off when moving. Air in the system or reverse rotation water pump/fan on a standard rotation setup/ standard rotation on reverse rotation setup only other possibility is blown head gasket.
2X Kacy's primary suggestion. Do this simple test to check for fouled watersides of the radiator cross tubes. It will also confirm adequate water pump flow and t-stat operation;
1. With the engine cool, open the radiator cap. Leave the cap off.
2. Drain off enough coolant so that the level in the radiator is about halfway down the tubes.
3. Start the engine and allow it to reach normal operating temp.
4. When normal operating temp has been reached, look down through the neck and observe whether coolant begins to flow out of the ends of the
tubes and into the tank. If it does, that would confirm that the t-stat is opening at the correct temp.
5. Next, have your assistant rev the engine up/down while you observe the level in the outlet tank. When the RPM's are increased, two things should
happen
a. The volume of water issuing from the tube ends should increase substantially and
b. The level in the tank should fall noticeably
6. When the RPM's are allowed to return to normal, the coolant should quickly rise back up to the initial level.
While you have the cap off and the level is down, inspect the ends of the tubes carefully using a flashlight. Note whether there appears to be any deposits that have formed. If so, you can be almost sure that the watersides have become coated all the way across. Sometimes, while there is sufficient flow through the tubes, heat transfer is inadequate due to the film of mineral deposits.
