Coolant overflow boiling// thick foam - 2004 Chrysler Pacifica

Well, I would say that the radiator would be next on your list, but usually the overflow is attached to the radiator, so that should be impossible to boil before radiator. How does the temperature gauge look? Even if any parts are hot to the touch that doesn't necessarily mean they are overheated. It may be that the system is still working out bubbles, hence the overflow boiling over.

is the cooling fan coming on when it is hot?

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Its normal for the car to have antifreeze going to the overflow tank if thats what your asking.

If the thermostat is not opening, the engine will keep getting hotter as you drive. You could be low on coolant and have air pockets in the system which would cause the temp gauge to not be accurate. Is the radiator fan coming on when the tank is boiling ?

I just had this , it's was a cracked expansion tank on mine, got it replaced and it's fine now. Jara13

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

When sitting still, yes both fans on a car equipped with two fans will run. The temperature ranges you posted seem within acceptable range. That said, the water (coolant) should not boil at those high temperatures, but some overflow is expected/normal. Ensure that you are not adding pure water to the radiator and instead are premixing it with antifreeze coolant or adding premixed 50/50 that you can buy at auto stores as this will keep the water from boiling and freezing.

As an added point of information, the hose leading into the overflow jug is two-way, meaning that when the coolant in the radiator cools down enough, it creates a vacuum and pulls coolant back into the radiator.

Zach

REPLACE THERMOSTAT AND COOLANT OVERFLOW PRESSURE CAP IF THERMOSTAT STUCK CLOSED IT WILL CAUSE ENGINE OVER HEAT COOLANT BOIL OUT AT THE COOLANT OVERFLOW RESERVOIR.

The thermostat is slow to open because the coolant is too thick from the oil. The coolant is thick because oil is getting into the coolant. It sounds like you have a leaking head gasket or a cracked head.

thermostat