1997 Pontiac Sunfire 2.4L

I have a cooling system puzzle to solve.

My cars temperature gauge isn't showing it is overheating but it is running hot. If I open the hood after trips longer than about 10 minutes, I can see steam coming out of the coolant reservoir tank cap. If I get up to higher speeds, the engine really heats up, the coolant boils and all of it leak out of the reservoir tank cap.

I took the car to a mechanic and was told I have a cracked head gasket that is causing coolant to leak into the first combustion chamber. The thermostat is also part way stuck.

I'm trying to decide if I should replace the thermostat first or use a head gasket sealer. Which would make more sense to cause the cooling system to act this way? The car is on its way out the door, is why I willing to risk using a sealer but I'm not really sure the cracked gasket is causing the problem and would rather not risk it at all.

Some other relevant information.

I don't seem to be losing coolant on trips where I stay at lower speeds.

I did a combustion gas detection test and it was negative.

I did a coolant pressure test myself and it held pressure for 30 minutes.

I blasted the heat and it was really hot, didn't seem to cool the engine down at all.

I blasted the AC and it seemed to increase the temperature slightly.

Thanks for the help!

  • Did you already replace the radiator cap?
    – devopsix
    Feb 24 at 7:02
  • It doesn't have a radiator cap, it has a coolant reservoir/expansion tank cap. No I haven't replaced it.
    – arock
    Feb 24 at 14:53

2 Answers 2


You have already decided you are not going to invest in head gasket replacement. So make sure the radiator fans are working properly, make sure the radiator is not restricted. Replace radiator cap and thermostat and see what happens. If you have problems after that then sealant might be an option, but not until all other things are taken care of. Sealer should only be used as a last resort.


I did a combustion gas detection test and it was negative.

Then I'd say you don't have a head gasket leak. My car had one and it was not causing overheating. It was overpressurizing the cooling system, causing some misfires at idle because of coolant in a combustion chamber, and creating a bit of mayonnaise on the oil cap.

I'd personnaly change the thermostat before anything else as a stuck thermostat could cause overheating.

I blasted the AC and it seemed to increase the temperature slightly.

That is probably because your AC condenser is near/in front of the radiator. In order to blow cold air an AC extracts calories (heat) and dissipates them through the condenser.

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