After a 20-30 minute stop-and-go drive today, I parked my car and noticed that coolant was boiling out of the reservoir neck. The water pump, radiator cap, and coolant were all replaced in February 2004 (and the pump appears to be working fine). From what I've read in searches, today's random overflow could have been caused by:

• Radiator cap: The cap looks fine, I guess. It was a little gummy, but the spring still compresses. I really don't know what to look for though.
• Thermostat: Never been replaced, although I do have a 160° stat just sitting around here.

I'm hoping it's the thermostat, but just in case, what are some other possible causes of the problem?

Thanks

 

Sounds like the thermostat to me. Most of the new ones go out while in the fully open position to prevent overheating. Its the cheapest part and it'd be my first thing to change out.

 

or your radiator could be plugged up.

 

If the thermostat went out in the open position, why would my coolant get hot enough to boil over?

 

It is possible for a car with no thermostat to overheat because the water flows through the radiator so fast it doesnt get a chance to cool off.

If yours is stuck open, it might be possible for it to behave similarly but I doubt it because the thermostat would still slow down the flow significantly allowing the coolant time to cool off in the radiator.

Are you sure your radiator is not plugged up?

 

Not sure. What should I be looking for to tell if it is?

 

change the radiator cap 18 psi. burp the lines and make sure fans are kicking on.

 

Good question. I used to have an 88 Iroc and when it started overheating I replaced the thermostat and the radiator cap first. That didn't help. The water pump was less than a year old, so I got a new radiator for it and that fixed it. I assume it must have been plugged up. As mentioned above, also make sure your electric fans are working as they should.