So my dad and I replaced the thermostat, outlet housing, and radiator within the past month (all went bad at once) So ever since replacing the system, we have been noticing that as we put coolant in the system, coolant continues to leak through the overflow hole at the top of the overflow tank. I know it's supposed to be a sealed system but continue to think that air is getting in the system allowing more pressure to build and push the coolant to the overflow. I know it's not the head gasket as the oil is fine and no signs of water/coolant in the oil. What else could it be???

  • did you replace or otherwise test the cap? It's rated to a certain pressure, which once exceeded, will release coolant to the overflow tank. If it has weakened over time, it's no longer performing to specifications.
    – fred_dot_u
    Oct 30, 2018 at 14:42
  • Welcome to Motor Vehicle Maintenance & Repair! Oct 30, 2018 at 18:33
  • yes we replaced the cap twice
    – Chrispy
    Oct 31, 2018 at 15:07

2 Answers 2


As fred_dot_u stated, the reason your car's coolant is overflowing as it is is due to the radiator cap no longer functioning properly. My sister's car, a 2004 Dodge stratus...same vehicle, was in to a mechanic recently for engine performance issues, however, the mechanic also noted that he, for whatever reason believed the engine was overheating when it was not and claimed the boiling over coolant was a result of that. In reality what was occuring was that the coolant was absorbing more heat from the engine after it was turned off and, unable to maintain the pressure normally required by the cooling system, was overfilling the reserve tank.

All in all a radiator cap replacement should fix your problem as it did on my sister's. Best of luck to you.

  • Thank you for that. I did forget to mention that my dad and I replaced the cap twice. and still having the same problem
    – Chrispy
    Oct 31, 2018 at 15:07
  • What I would do next then would be to basically double check that the head gasket isn't the culprit here with a block tester. In some cases the head gasket can develop a very small leak between the cylinder and the coolant passages causing the excess bubbling in the overflow tank. If this leak is small enough it could cause the bubbling up in the tank and no other apparent symptoms. This would be my next step.
    – Techlord
    Nov 1, 2018 at 4:30
  • Just as a chaser to my last comment if you find the block test comes out favorably then another thing I can think of might be an issue would be too much residual heat in the engine itself after powering off. During the normal operation of the engine this isn't an issue as the coolant is being cycled by the water pump, however, once the car is powered down the water pump is no longer operating the extra heat not yet absorbed from the engine by coolant begins to work its way into the coolant. This additional heat can cause the coolant located in the block at the time to reach its boiling point.
    – Techlord
    Nov 1, 2018 at 4:38
  • When this occurs the coolant will turn into a gas and work it's way back to the overflow tank and cause the coolant to expand and bubble out the tank resulting in a leak. In some other vehicles these issues of residual heat percolation have been know to blow out head gaskets from time to time and as such manufacturers have implemented stratagies, such as allowing the cooling fans to run even when the car is off, to prevent these issues. I'm not sure if this is an inherent issue with these particular cars but that's my theory thus far. Of course, start with the block test and go from there.
    – Techlord
    Nov 1, 2018 at 4:43
  • So if it is the head gasket. How is your recommendation of Blue Devil?
    – Chrispy
    Nov 1, 2018 at 15:47

Those cars are notorious for head gasket failures, and depending on the leak location you may not get water moving into the oil. The cylinder pressure even before ignition is about 10 times higher than the coolant pressure, so the tendency is for combustion gas to leak into the coolant, forcing coolant out through the overflow.

You also could have an air pocket that wasn't been completely removed when you refilled the system. Many of the Chrysler vehicles of that era required an additional accessory to be attached to the top of the coolant reservoir when you filled it in order to build enough elevation head to push the water into all of the air pocket areas.

  • we continue to try and release the pressure with the attached outlet housing(bleeding the line of air)
    – Chrispy
    Oct 31, 2018 at 15:08
  • In the past I had a Beretta with the 2.8 V6 with the same symptoms as you describe. In my case it turned out to be a cracked block (!), but a failed head gasket or cracked head could do the same thing. A block tester (as others have described) is a good test or you could try a radiator leak kit which Autozone rents for free. It will let you pressurize your cooling system without the engine running. Aug 7, 2023 at 23:14

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