My Honda has been overheating when it is in idle. The temp gauge will jump to max really fast. After driving it for 5-10 mins a white sweet smelling smoke will come from the coolant reservoir. I checked my coolant after driving around and it was boiling in the reservoir. I flushed my coolant and changed my temp sensor. What could be causing this overheating problem?

  • does it overheat while moving?
    – rpmerf
    Mar 17, 2016 at 17:54
  • How much mileage does the Civic have?
    – mr_tuner
    Mar 17, 2016 at 17:56
  • It does over heat while moving but at a significantly reduced rate. It will be at normal temp the. When I stop at a red light it will start overheating and smoking. Also there is no leak into the ground so if there is a link it would be I to the engine. I just hope that it is not the head gasket.
    – Marty
    Mar 17, 2016 at 18:32

2 Answers 2


If this is all while you are parked and idling the car, the first place to look is the cooling fans directly behind the radiator. While moving, the motion of the car forces air through the radiator cooling the fluid flowing through it. When stopped, the ECU should be starting one (or two depending on the car) electric fan on the back of the radiator. This pulls cool air across it accomplishing enough cooling to prevent the boil-over you're seeing. If the fans are no longer working, or the sensor that turns them on is bad, then no fans, and the car boils over.

Also, check the level of the coolant when the engine is warm, but before it has boiled the coolant. The overflow tank should be marked as to the proper levels.

Do you know what the ratio of coolant to water is in your car right now? It should be 50/50. If there is too much water, it will boil at a lower temperature.

An easy way to test this is to start the car, and just let it idle. If you can feel the top radiator hose getting hot, and no fan, then you have narrowed down the problem. If the fans don't come on, try looking for voltage on the connector that runs the fans. If there is a voltage present when the car is warming up, then the fan is bad. If you can feed 12V directly to the fan and they run, then the fans are fine and either the sensor, the ECU, or perhaps a wiring issue is preventing the fans from being told to come on. One additional thing to check is that the ground is good between the fan and the battery. Try checking the resistance between the negative connector on the fan and the battery negative. It should read less than 1 or 2 ohms. If it's more than that, you have a bad ground. I learn this one the hard way. Replaced a perfectly good circulator fan only to find out the new one did the same thing. The problem was a bad ground at a cold solder joint where someone had tried to splice extra length into the harness.


Here is a Popular Mechanics article that discusses the sweet smell coming from the Civic.


[The sweet smell information is below.] All credit goes to Popular Mechanics.


WHEN: After the engine has warmed or possibly even after it's shut off for a few minutes.

THE CULPRIT: Coolant containing sweet-smelling (but toxic) ethylene glycol is leaking from somewhere. It could be coming from a radiator or heater hose, a failed intake manifold gasket or cylinder head. It might be coming from a leaky radiator cap or the radiator itself, especially if you smell it outside the car. A strong odor inside the passenger compartment probably means a bad heater core.

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