You have a hot spot in the engine or a cylinder leak(head gasket)

What engine do you have in the 2004?
2.3l, 3.0l or 4.0l were the options

After engine has cooled off remove rad cap and make sure it is full to the very top
Leave cap off and start the engine
You will get an initial surge of coolant coming out of the cap but it should stop after a few second
If it doesn't then you have a cylinder leaking pressure into the cooling system, blown head gasket or cracked head

If coolant stops coming out then you may have an air "dam" in the heater hose or head
Shut off engine
Loosen and remove either heater hose at the firewall, should have coolant coming out heater and hose both, if not add more coolant to radiator until coolant is at the hose, reconnect hose to firewall
Start engine
Coolant level should drop now, refill to the top and put rad cap on
That should take care of the hot spot

 

 

Sorry 2000 Ranger

3.0l can be a bit of an odd duck with over flow at the cap
Take rad cap off cold engine, coolant in rad should be low if reservoir is full

Start engine and see what happens, it should not overflow
Add some coolant or water while engine runs, see if this causes it to overflow or you see bubbles in the coolant
Also smell the air at the rad cap, see if it smells like exhaust, that would mean a cylinder leak

There is also a Glove Test you can do to confirm a cylinder leak, its free and easy


Yes, if you have a small coolant leak the upper engine can end up with air in it, and the water pump can pump fluid but not air, if there is enough air then it blocks coolant flow in parts of the head
When you shut off the engine the water pump stops, so some of the coolant can run into an are the is very hot, coolant is instantly vaporizes and expands to 4 times its volume, this pushes open rad cap and sends coolant and then air(the bubbles) over to reservoir.
This is a long shot but possible

If your 3.0l has been overheated in the past then a blown head gasket is far more likely

 

 

 

Good, doesn't read like a blown head gasket

Also test or replace rad cap, but you do have a coolant leak

How "overflow cooling system" works:
Rad cap has TWO valves inside, the larger one that holds in pressure as coolant expands, and the a smaller one in the middle that SUCKS coolant back in as engine/coolant cools down

All liquids expand when heated, that's how a pressure cooker works

So as the engine heats up the coolant, it expands in volume
This creates the Pressure inside the system, this pressure raises the Boiling point of the coolant, which is good
When the pressure inside the system reaches Rad Cap rating, say 15psi, then rad cap is pushed open and coolant flows over to the reservoir until pressure is under 15psi

After engine is shut off coolant cools down and SHRINKS back to original volume
If any coolant flowed over to the reservoir then pressure in the system will drop below 0psi to -1psi, at that point the smaller valve is PULLED open and coolant is SUCKED back in to the radiator.

So pretty simple system, and it is Self Purging of Air

Air floats to the top in coolant, so during circulation of the coolant any air should end up in the top of the radiator.
(heater hoses are also a high point so air can be trapped there, and if there is enough air it can block coolant flow)

So air is at the top of the radiator, as pressure rises the rad cap opens and the AIR is sent over to the reservoir FIRST
The reservoir is designed to be vented, its not a sealed container
And the overflow hose's end is at the BOTTOM of the reservoir

So any AIR that comes over from radiator, as engine heats up, bubbles up to the top of the reservoir and is gone
As engine cools down COOLANT is sucked back in to replace the air
So after full cool down radiator should be FULL to the top with coolant, no air at all in radiator
Self Purging

So if you open rad cap and see air at the top then you have a leak
What happens is that on cool down air is being sucked in from the leak point because it easier to move than the heavier coolant from the reservoir

Leak could be in the overflow hose, check it

You may need to rent a cooling system pressure tester and test system cold by pressurizing it, and then looking for dripping coolant
A leak near a warm spot on engine never drips to the ground, it evaporates before it does
Pressure tester also has a gauge, so you can see if system is holding pressure, of course it won't, if there is a leak

After you shut the engine off circulation stops so coolant in the heads will heat up more so expand a bit more as it should, this increases pressure in the system so rad cap will open
The bubbling after shut down means there IS air at the top of the radiator and that's what is coming over after shut down, its not "boiling" just air that is in the radiator, but there should be NO AIR in the cooling system