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
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