Two months ago, while driving my Saturn around in stop and go traffic, my coolant boiled over. After taking it into a shop, the mechanic deemed that the thermostat was bent, and a simple replacement would fix the problem. Flash to yesterday, when it happened again, for the first time in two months. The strange thing is, not once has the temperature warning light gone on, or even surpassed the halfway mark on my temperature gauge. When it happens, the coolant literally flows out of one of the caps. My suspicion is that there is a pressure leak somewhere, making the coolant system boil lower… is this a good hypothesis, or am I off base?
Try replacing the pressure cap with one of the same pressure.
You may have a small leak somewhere that lets air into the system. When the thermostat was replaced, the air was bled out and that’s what fixed it (momentarily), not the thermostat replacement. Bleed the air out again, and it may be okay again for a little while. You need to have a pressure leak down test done and find the leak.
If the coolant is not circulating properly due to a bad water pump, you could get very very hot coolant in the engine but still be cool around the temp gauge, theoretically anyway.
If you have a head gasket failure, it will look like a boil over but its really hot exhaust gasses escaping, but that would happen all the time, not just once every two months.
I’d start by replacing the coolant overflow cap, you’d be surprised how often this fixes cooling systems.
Hi all,
The coolant was definitely flowing out of the overflow cap last night… leading me to believe that this was probably the source of the problem. I wasn’t sure if it was just the fact that the system had built up too much pressure, or if it was the cap itself.
Thanks so much for the advice! I went down to the nearest Saturn dealer (once I found one… not too many left I’m afraid) and bought a new coolant overflow cap this afternoon. I’m going to replace it and see how it works… if I have the issue again I’m going to take it in for a pressure test. For now though, I’d like to see if the $10 cap will do the trick 
Thanks!
-Dave
You can checkk the radiator from the back side. If you can’t see enough, you might want to lean the shroud back to see it. If you see any fins missing between the tubes because they corroded away, you need a new radiator too. If you see white or green coloring on the radiator fins, you may need a new radiator. If the car is 13 years old and you see white or green, change the radiator.
Be aware that not only can these common parts be obtained through any GM dealer, they can also be obtained at any parts store, usually cheaper.
With so many dealerships closing and brands being totally eliminated I thought this information might be of value to you. You don’t need to go to a Saturn dealer.