I have a weird problem. I recently changed the radiator and flushed the whole coolant system during replacing the radiator. I also flushed the heating core by itself. The thermostat was recently changed and it doesn't seems to be stuck open because if the car is sitting it warms up properly. The coolant temperature sensor was also changed recently because the temperature gauge would start in the middle and that seemed to be the solution to that problem.

The problem: When I start the car and leave it sitting, it warms up properly and stay at 1/2 of the temperature gauge. However when I start driving the temperature gauge drops a lot. Most of my commute is on the highway and most of the time the temperature gauge stays at 1/8. This mainly affects the heat, right now if I leave the car sitting the heat works great but once I start driving on the highway it is not as hot.

I talked to an employee at Autozone and he told me to make sure I burped the engine. From what I am reading online the car would be over heating if it was not properly burped. But it can't hurt to try, so I plan on getting a lisle funnel soon.

Any other solutions or ideas of what the problem might be?


  • 3
    It sure sounds like a stuck thermostat. When the car is sitting, you don't have air running through the radiator, so I would expect it to warm up some.
    – JPhi1618
    Oct 30, 2015 at 15:26
  • Did you change the seal along with the thermostat?
    – Zaid
    Oct 30, 2015 at 16:31
  • 1
    And, while this may seem obvious as not an issue, are you sure the thermostat was put into the engine correctly? One which is upside down will not function as it's designed to. Just one more thing to eliminate. If it was put in right, I'm with the rest of everybody in that I believe the thermostat is stuck open. Oct 30, 2015 at 19:14

3 Answers 3


If the thermostat is stuck open, you would get the symptoms you describe. A thermostat stuck closed will cause overheating.

  • Hmm if the thermostat is stuck open wouldn't it not warm up at all?
    – emmanvazz
    Oct 30, 2015 at 15:41
  • Also the thermostat is only months old. I would say max 6 months but I guess it could still be stuck.
    – emmanvazz
    Oct 30, 2015 at 15:44
  • Yes, it works just like the thermostat in your house, it tries to keep the engine at a constant temperature. In your house if you leave the windows open the boiler will still warm the rooms up, but it will not be as quick or as warm as if you shut the windows. With the thermostat on the car stuck open, the radiator will be acting like the open windows, especially when air is flowing over the radiator when moving.
    – HandyHowie
    Oct 30, 2015 at 15:47
  • Okay I guess I will try to change it and see what results I get. Thanks for your help.
    – emmanvazz
    Oct 30, 2015 at 16:07
  • As @zaid says make sure the seal is in good order
    – HandyHowie
    Oct 30, 2015 at 17:07

One possible explanation is that some coolant is bypassing the thermostat altogether.

Depending on the thermostat and seal arrangement, this could happen if the thermostat isn't properly seated in the surrounding seal, or if the seal is compromised and not doing its job properly.

It would also explain to some extent why the thermostat seems to work when the car is stationary but not when the car is moving. In highway driving, the water pump runs faster, pressurizing the coolant more, which allows more cooalnt to leak past the thermostat.


Did you replace the radiator with OEM or aftermarket? It almost sounds like the new rad has more surface area and it cooling at speed better than the old one did.

  • I got an aftermarket one of eBay. It fit perfectly but maybe that is true. Any ideas as to how to correct this?
    – emmanvazz
    Oct 30, 2015 at 15:40
  • 4
    The thermostat should control the flow of coolant into the radiator to regulate temperature, so it shouldn't matter how oversized the radiator is. It will be a problem if the radiator is too small however.
    – HandyHowie
    Oct 30, 2015 at 15:49
  • I would like to point out that if the radiator were undersized, the motor would overheat instead of being too cold.
    – sleblanc
    Oct 30, 2015 at 17:59

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