I have a 2000 Sebring and the overflow boils after I drive it a little bit. Last week all the water drained out of the radiator but the overflow was full. Once the cooled sufficiently, I removed the radiator cap and filled it with water and all was fine, but the overflow continues to boil. Any ideas where to start checking?

 

in no particular order... :
Bad cap on overflow bottle not allowing system to build proper pressure ( buy new cap )
Water pump not circulating water ( check for leak from weep holes, check oil dipstick for discoloration)
Air pocket in system misleading you to believe its full when in fact it is not ( bleed air out of cooling system)
stuck thermostat not allowing proper circulation ( change thermoststat)
improper mix of coolant/water in system lowering the boiling point of the mix ( replace with proper ratio of coolant to water 50/50 or 60/40 coolant to water)
Blown head gasket

 

What is a weep hole? Dumb it down for me...

 

What is a weep hole? Where do I look for leaks? The cap on my reservoir is fairly new. How hard is it to change the thermostat?

 

Spend a few bucks and buy a repair manual like a Haynes manual specific for your model. Read through it in the relevant chapters. Then you can decide whether you and your toolbox are up to the task.

Your report of adding water to the radiator leads one to conclude that you may not understand the basics and the need to use a proper mixture of antifreeze and water in the cooling system. That alone could account for the boiling over but there are many possible causes of overheating ranging from very simple to very complicated.

If you'll take the time to study up on the problem via a book on the subject you will either be able to fix it yourself or, at least, discuss intelligently with a professional repairman.

 

what engine is in the car, different model engines have different locations for the weep hole.
A weep hole is a hole designed to leak your coolant to the outside in case of a waterpump failure, this allows you to see the vehicle is leaking something and to investigate and make repairs before it can cause further damage. Obviosly if the waterpump fails, no coolant circulates and the engine overheats...if left unchecked it can blow a head gasket or spin a bearing...IE the engine is operating at a much higher temperature than designed for and placing a strain on all the components...
Weep holes are USUALLY located on the bottom of the waterpump itself, so when you see the coolant boiling, look on the ground under the water pump and see if there is coolant leaking there or down the side of the engine block.... if you have a 2.7 the weep hole is actually located on the side of the block where the lower radiator hose connects and is part of your thermostat housing, look and verify no leaks are present in that location...

Your best hope is your cap is bad, or your thermostat is stuck, or your mix isnt right or your system needs to be bled... in any case DO NOT DRIVE IT THIS WAY, especially if its a 2.7 unless you want to end up replacing your engine...park it until you can find the cause of this boiling over before it does more extensive damage to your engine.
pull your oil dipstick and see what the oil looks like, is it a thick sludge ? milky color ?

you state your cap is almost new... what previous repair was made that replaced it ?? ( your cap should be 16 years old )... if its old it could be as simple as a worn out cap not keeping the right pressure in the system which is lowering your boiling temperature and creating the problem.

When its boiling, what does your temperature gauge read inside the car ??
When its boiling, turn the heat on HI, is there hot air coming out ?? ( should be extremely hot, if not air pocket or low coolant )

FIRST STEP I would do is replace the cap, no matter what it looks like ( unless you have a cap tester, which I've heard exists but have never seen because most likely the tester is more expensive than simply buying a new cap)
after replacing cap, follow procedure to bleed air out of your system, this is extremely important , especially if its a 2.7
ENSURE you have the proper mix ratio of water to coolant ( 50/50 or 60/40)

AFTER the above has been done, if you still have a problem, we can go FROM there... but at the moment those things MUST be done in order to diagnose and get to the real problem and rule out the simple ones.

 

If it's a 2000, it won't be a 2.7L engine. That came in in 2001.