I have a 1998 Pontiac Grand Prix and I have noticed that when I drive it for about 15 to 20 minutes and then park it and turn it off my antifreeze is bubbling. My thermostat does not say it is overheating but it smells hot and my low coolant light keeps flashing. Any suggestions on what it could be?

  • Your thermostat doesnt say it is overheating? Do you mean temperature gauge? – Steve Oakes Jul 19 '16 at 20:05

A simple cause of boiling coolant is a radiator cap that doesn't hold pressure anymore. The cooling system is designed to be under a small amount of pressure, and that pressure increases the boiling temperature of water to the 250 degree neighborhood. A working cap will hold 12-15psi, and without it holding pressure, boiling happens much sooner, depending on the ratio of coolant you have.


Coolant boiling over usually indicates poor coolant circulation. Either your water pump is starting to fail (they usually leak a little bit when they start to fail) or your cooling system is partially obstructed. Here's things to check:

  • look at the area near your water pump for small leaks (indicates worn out water pump)
  • check your coolant level in the plastic tank (add coolant if it's below max)
  • when the radiator is completely cold, open the radiator cap to verify that it is filled all the way up. Pay attention to the colour and condition of the coolant. Brown stuff floating is bad.
  • because it's a GM, it most likely uses Dex-Cool (redish colour) instead of "regular coolant" (greenish colour). These don't mix. Any chance your cooling system got topped-up with the wrong coolant?
  • Could you please explain the relation between boiling coolant and poor circulation? – I have no idea what I'm doing Jul 11 '16 at 7:56
  • @IhavenoideawhatI'mdoing If pockets of fluid are being exposed to a hot part for extended periods of time before reaching the radiator for cooling, they could surpass their boiling point. – James T Jul 11 '16 at 13:35
  • @JamesTrotter and cause high temperatures reported by the ECT sensor, no? – I have no idea what I'm doing Jul 11 '16 at 14:08
  • @IhavenoideawhatI'mdoing Depends if the coolant that has been heated is managing to reach the sensor I guess. – James T Jul 11 '16 at 14:11
  • @JamesTrotter But it is going to reach it if there is any circulation at all, as it's usually placed near the thermostat. And if it's slow, coolant will be HOT. Which means high temperature will be reported, which isn't the case here as I understand. Unless there is an air pocket or the ECT sensor is lying. – I have no idea what I'm doing Jul 11 '16 at 14:18

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