My Car Overheats in Idle & Cools Down When Driving

Check your fan belt and fan blades if you have a mechanical cooling fan without a clutch (it only moves with the pulley). If you have a belt problem, then you'll know it right away because the water pump won't work. If the fan blades have gotten bent or tweaked, they might not pull the requisite amount of air.

Check your fan clutch if you have a mechanical cooling fan with a fan clutch. This type of fan will move, albeit with some resistance, when you turn it by hand with the engine off. Start the car and remove the radiator cap so that you can see down into the radiator. Check the water level to make it's at or near the top of the radiator. Pay attention to the temperature gauge and wait for the engine to reach operating temperature.

Look into the radiator and watch for water circulating through from the engine. If the engine starts to overheat but you don't see water flow, then have an assistant rev the engine up for a moment. If water doesn't move, or suddenly rushes into the radiator, then you probably have a problem with the thermostat or water pump, or an internal water leak. Replace the radiator cap if the water starts to flow.

Watch your engine cooling fan. If your fan clutch doesn't engage when the engine starts to overheat, then the fan clutch is likely shot. Replace the fan clutch.

If the electric fan doesn't come on when the engine reaches normal operating temperature, then you either have an electrical problem or the fan is malfunctioning. Shut the engine off and try to spin the electric fan blades with a screwdriver. The fan should spin freely and with almost no resistance whatsoever. If it's difficult to move or grinds to a halt, then replace it. If the fan spins freely but doesn't turn on when needed, proceed to the next step.

Check the fan fuse or relay. If the fuse is blown, then replace it. Start the car and allow it to come up to temperature; recheck the fuse to see if it's blown again. If it is, then you have a dead short in the fan motor or in the electrical system. Check your reference material on how to test the relay. If it doesn't work then replace it.

Cut two wire leads, each about 3 feet long, and strip about 1/4-inch of insulation from the ends. Unplug the fan, and insert one end of each wire into each of the terminals in the plug. Hold the other end of one lead to the negative battery terminal and briefly touch the other end to your positive terminal. If the fan comes on then it works and your problem is in the electrical system.

Connect a test light to the chassis side of your fan connection plug, the one attached to the body. If the light doesn't illuminate when the engine overheats but the fuse is good and the relay works, then test the fuse and relay terminals in the fuse box to make sure that they're getting power. If the relay's trigger terminal (check your reference material) doesn't illuminate when the engine overheats, then the water temperature sensor is bad.