When you stop, so does the air flow from outside. As a result, the temp under the hood raises. After it hits a certain temp, the fan clutch is to tighten causing the cooling fan to spin faster, drawing outside air through the radiator to cool the engine. If the clutch fan isn't working, it will heat up when you are sitting still.
Let me know if you have other questions.
Monday, February 8th, 2010 AT 7:18 PM