I have a 2000 acura 3.2tl and my reservoir tank keeps boiling over and steaming quit a bit. My engine isn't over heating and it doesn't happen right away. I have flushed my radiator and changed my thermostat. I don't think that it is my heater core because the heater still runs fine. What else could be causing it to do that?
I would start by replacing the radiator cap. If you can get the system (and the cap) pressure tested, this will help narrow it down.
However, I've seen more than once a cap that tests good on a tester, but doesn't seal properly on the radiator neck. Cheap enough to buy a Stant at the parts store and see if it makes a difference.
I had the same problem. My dads 1997 Subaru Legacy was doing the same thing. Try checking the radiator fans. See if they are damaged or unplugged, or disconnected in any way. It could be the fuse for your radiator fans. I would bet on that. If the fuse blew, the fans, quite obviously, won't work, so the reservoir tank heats up, and boils over. Pay attention next time to when it heats up and boils over. If it is fine while driving, but boiling over when you stop or idle, it is most definitely the fans, or the fuse for the fans, because air blows through and cools the radiator and the reservoir tank. when you stop,if the fans are broken, or the fans fuse blew, no air blows through and cools, causing it to overheat and blow over. Otherwise, check for leaks anywhere near the reservoir tank, or coolant hose. i hope that helps:)
I agree with SteveRacer your radiator cap is probably no good. You can take a look underneath it to see if there is any cracking with the rubber seals. If there is then this is probably where your problem lies.
You should also inspect the seat where the cap seals against inside the radiator for any debris that may be allowing the coolant to bypass those seals. Not likely but might save you $5
The reason a bad cap can allow the water to boil is because the pressure the cap is designed to keep in the cooling system makes it more difficult for the water molecules to escape as a gas. In fact if you placed the coolant in a vacuum it would boil sooner. here is a handy chart if you ever need it. It has some other useful tools and info in it.
One last point is, make sure your radiator is topped off before you go anywhere because if the coolant level gets below the heads the heat buildup can cause damage. You will also want to be cautious in general about water levels since temp sensors do not work in air very well.