== == Sometimes it is not actually "boiling", but bubbling. Start by getting a new radiator cap. It is the first thing you should do, and is easy. *MAKE SURE YOUR CAR IS NOT HOT WHEN REMOVING A RADIATOR CAP* When air enters the closed/pressurized cooling system, the air ends up in your reservoir tank and bubbles the top. Sometimes(depending on vehicle) it will even exit the reservoir tank through an overflow hose and appear to be a leak. Hope this helps! Chris not enough water or coolant in radiator.
AnswerThis is a symptom of an overheating engine. See "Related Questions" below for an excellent trouble-shooting guide to overheating problem.
it is normal, when the coolant is hot and pressure built, the pressure cap allows the coolant to drain to reservoir to reduce pressure in the cooling system But, if it continually does this whereby the reservoir overflows and the vehicle overheats due to a lack of coolant actually in the radiator when cool, there may be an overheating problem or blown head gasket on the engine.
1) Your engine is overheating and the coolant is boiling. Or 2) Exhaust is leaking into the coolant, probably through a bad head gasket or cracked head.
could be cylinder head gasket gone
There may be other reasons for the vehicle to be overheating but yes if you have let the reservoir run dry then it wont have suction to draw fluid back into the radiator so you will have to fill the radiator as well.
Failed thermostat.
Check the coolant level in the reservoir. If that's up and the vehicle isn't overheating, you probably have a bad sensor.
Overheating due to coolant going to the reservoir and not coming back to the radiator because of the bad cap.
Bad gasket or overheating of the engine. Check cylinder compression and seals.
one reason this may happen is as simple as the cap on your cars coolant reservoir is stripped or improperly screwed on. when this happens it doesn't allow pressure to build or hold.
Tough to narrow it down with such little detail, but it sound like your car's thermostat may be malfunctioning and not controlling the temperature correctly.
if the coolant in the overflow rises and overspills when you shut the car off or is boiling in the reservoir