I have a 2003 honda civic ex with 110k miles that over heats after freeway speeds. The water pump and timing belt were replaced at 100k miles.The thermostat was recently replaced as well.

The interesting thing about this situation is that the car doesn't heat up when just driving around town. It is only noticed after 30 min+ of freeway speeds when the car is promptly stopped at a stop light or in traffic congestion. When I see the temperature gauge shoot up to hot (it moves faster than the second hand of a clock), I speed up if possible and it promptly falls back down to the normal operating temperature.

I have also noticed when the car is idling that the hot air blows cold air up until the car is moving again when then it returns to hot air. I have checked the coolant levels multiple times and it is normal.

Any suggestions would be greatly appreciated, I have no idea what to do next.

  • do you hear the electric fan at the radiator turn on when it overheats? It isn't needed at travel speeds, but should be coming on as you slow down if the coolant is hot enough.
    – Tim B
    Mar 18, 2016 at 19:16
  • I had the same thought, but if it's the electric fan, why does the heat run cold when at idle? If it were circulating correctly and not being cooled by the fan, I would think the heat in the car should get hotter, not cold when at idle. I'm thinking it's the water pump is on it's way out. Perhaps it's not pumping enough when the engine is at idle, but at highway speed it does. That would also explain the rapid rise when stopping quickly after highway. If there is not enough flow, all that heat from highway just heats the coolant nearby since the water pump isn't moving it well.
    – cdunn
    Mar 18, 2016 at 19:19
  • How's the fluid level? You say the thermostat was recently replaced, did this only start after the thermostat was replaced?
    – Ben
    Mar 18, 2016 at 21:23
  • I have heard the electric fans turn on, but this has been when the car is not overheated. Next time it overheats I will be sure to listen for the fan. The fluid levels are good, but yes this was first noticed after the thermostat was replaced. The water pump makes sense with your explanation.
    – Ken
    Mar 18, 2016 at 22:03
  • Were you able to test the fan and get more data? Mar 20, 2016 at 13:18

1 Answer 1


IMO this classic low coolant level

Even if your coolant looks fine in your overflow tank it doesn't mean your cooling system is at appropriate levels or that there is not air in your cooling system.

Here is a perfect example of a similar situation.

When your car is cool, remove the radiator cap and check your fluid level.

If the fluid level is ok, start the car with the cap off. Have a friend rev your engine a bit and see if the fluid level drops and air bleeds out of the system. Fill the radiator accordingly. Wash, rinse and repeat this procedure until you can't add more fluid to the radiator and replace the cap. Once complete make sure your overflow tank is filled appropriately to the marks indicated on the overflow tank.

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