Hi, I have a 96 Honda Civic. I have been having issues for the past 2 months now with the coolant getting so hot it keeps boiling out of my overflow container. I have to fill the radiator every time I drive it. We have recently changed out the thermostat and then today we changed the water pump which neither took care of the problem. The motor was rebuilt about a year and a half ago and all gaskets were replaced at that time. There are no leaks in the radiator(this was also replaced within the last year) and the coolant only comes out of the overflow. About a month ago the overflow container got so out the the hose inside came off along with what looked like possibly a gasket of some sort but it had melted. I was able to put the hose back on but I wasn't able to figure out what the other piece went too. Could this possibly be part of our problem now since we have changed out everything else. We are out of ideas on what the problem could be. Please help.

Change thermostat first! Then change temperature sender!

I just today actually fixed a similar problem in my honda my problem turned out to be the Timing belt was loose and not gripping to turn the water pump causing all sorts of problems with the cars temp. and water temp. including the fan not coming on, it sounds dumb but it worked for me just push on the belt and if it moves its loose. quick turn of a wrench and fixed.. Good Luck

Answer could be a faulty thermostat,or it could be a sign that your head gasket is about to blow ,or last of all your timing could be out of sync.

Answer 2 from Inventus: It means your cooling system is funtioning properly. In a system having a coolant recovery tank, coolant in the radiator is always up to the brim, hot or cold. There is negligible or no air space. When coolant in the radiator expands sufficiently due to warming from the engine, it will squeeze past the pressure cap's bottom seal and flow into the recovery tank. (If no provision for such expansion was present, the expansion would rupture the radiator or your hoses.) Only coolant within the radiator is under pressure, and because of this pressure (together with the elevated boiling point that the "anti-freeze" permits), it normally does not boil. But once past the pressure cap's bottom seal, the overflow is at atmospheric pressure and therefore boils.
This boiling is usually unnoticed after a short, i.e., local, trip because the cooler coolant already in the recovery tank quenches it. But after some highway driving the influx of more hot overflow heats up all the coolant in the recovery tank to the (unpressurized, i.e., "natural") boiling point.
As the engine cools when shut off, the contracting coolant in the radiator sucks back coolant from the recovery tank. Fluid in the recovery tank should never be below the "full hot" or "full cold" marks, lest air be sucked in.
-- BETTER ANSWER ==
Your cooling fans are not turning on. It is not normal for your overflow tank to boil like that. It is true that your radiator is overflowing into the reserve tank, but that means yourr adiaotor is boiling. Check for blown fuses or relays for your cooling fans. IF theya re fine. run your engine for about 15 minutes and drive. When you temp level is at normal operating temp open your hood with the engine runing and see if your fans are on. If they are, then you may have a bad thermostat or a plugged radiator, or a bad water pump. If the fans are not on, get your cooling fan switch replaced if your car has one. Check your temp sending sensor

The cooling system is overpressurized and is lifting the radiator cap off of its seat at about 14 psi. This will push coolant into the overflow bottle. the most likely reason the system is overpressurized is that combustion gases are leaking past a blown cylinder head gasket into the cooling system. To verify this, you can use a Lisle 75500 combustion gas leak detector kit. it is inexpensive and will show if combustion gases are leaking into the coolant.

Cracked Cylinder head or cylinder head gasket blown. Bet you filled it up with cold water when it was hot! Check your oil level, see if it is going up (water getting in) Remove the rad cap start your engine and if it blows the water out then i'm probably right.

CHECK COOLANT LEVEL STILL LOW.ADD MORE COOLANT.UNTIL RADIATOR OVERFLOW FULL AT COLD MARK.THEN CRANK ENGINE UP LET IDLE FEW MINUTES LOOK DOWN COOLANT OVER FLOW JUG ADD MORE COOLANT IF LEVEL DROPPED.WHILE ENGINE RUNNING KEEP EYE ON TEMPERATURE GAUGE.WHEN THERMOSTAT HOSE GET HOT.TURN OFF ENGINE WAIT A LITTLE WHILE TO KEEP FROM GETTING SCALD TAKE LARGE RAG PLACE OVER OVERFLOW JUG RADIATOR CAP.OPEN CAP A LITTLE A TIME.IF COOLANT TRIES BOIL OUT.LET VECHICLE SET A WHILE LONGER.ADD MORE COOLANT.WHEN COOLANT LEVEL CORRECT AND ALL AIR OUT THE COOLANT SYSTEM.IT WILL STOP OVERHEATING.LONG AS ENGINE RUNNING HOT TURN OFF THE ENGINE POUR IN MORE COOLANT UNTIL COOLANT LEVEL STOP DROPPING IN RADIATOR.KEEP COOLANT LEVEL IN OVERFLOW AT COLD MARK.DONT OVER FILL OVERFLOW JUG.

sounds like you have an airlock.with engine running and hot, undo the clip on the highest hot hose(wear rubber gloves)and vent off any air trapped.make sure the expansion bottle is full before.and 1/2 full after.

Are your fans working? If not thats your problem.. I would check your coolant flow make sure your radiator is not clogged and pump is working...

I want you to take a couple of spark plugs out and examine them. If you find Anti-freeze on them then you may have a blown headgasket. A very common problem of driving a car until it overheats. I really do hope im wrong. Since it is an overhead valve engine with the cam shaft above...its a 1000.00 to 1500.00 dollar job to have it properly repaired. From here out im just going to explain why this happens...
When a car overheats - be it a faulty thermostat...low coolant...or a faulty water pump or improper coolant mixture ratio. The engine gets so hot that it actually makes the top of the engine change shape... it actually changes enough to damage the head gasket. The head gasket keeps the explosion inside the combustion chamber. Keeps the water in its coolant jacket. and keeps the oil inside its ports. When the Headgasket fails. Sometimes coolant gets into the oil (makes a milky residue form on the inside of the oil filler cap...or makes the engine oil look like chocolate milk.) or oil gets into the coolant (makes your coolant have an oil slick on top) It can also cause water to get into the combustion chamber and when the engine is running - blast the combustion back into the coolant (causes the coolant overflow to overflow or spray) or oil gets into the combustion chamber causing the plugs to get fouled (dirty). worse case...all the above happens. I hope this helps in figuring this problem out...i really do hope im wrong about a blown headgasket . As even i tremble in fear of it happening to my car. Good Luck!

It could be, take the thermostat and boil it in a pot of water, if it don't open, it's a bad thermostat.