Originally Posted By: Warstud
When you park your car after a long drive, water will sit in the engine block and absorb heat from the metal. That's why your temperature gauge will always spike after you shut the engine down, and will quickly drop when you re-start the engine after it's had a chance to cool slightly. However, if your water pump is dead, then water won't circulate and the temperature needle won't drop as quickly when you re-start the car. So, run your car until it gets up to temperature or starts to overheat, and then park it. Turn the ignition key to the "on" position, and watch the needle. You'll probably see it rapidly rise; if not, then this alone could indicate water pump failure. Re-check the temperature after a couple of minutes, when the temperature needle just starts to fall. Start the engine; if the needle rises and the cooling fans are on, then you've got either a bad water pump or a clogged radiator. A needle that doesn't fall quickly or remains relatively stationary could indicate the same things.
by Richard Rowe, Demand Media
Thank for giving me better explanation. After car overheat in long drive then after I restart the car the temperature is back to normal temperature. Later, it will overheat after 10 minutes. I think I have problem with the radiator. How can I get rid of brown sludge inside the radiator?