Those that are old enough will remember that most vehicles used a Degas system until the 1970s
Radiators had a FULL LINE about 2-3" below the top of radiator, and a overflow hose that just ran down the side of radiator to dump coolant on the ground if it over flowed
Degas means you leave enough AIR in the top of the radiator for the coolant to expand when its heated to 170-220degF
Degas Bottles are the same setup, the bottle needs to be the HIGH POINT in the cooling system, just the top part of the bottle/tank, it will have a FULL LINE, just like the older radiators
This is so any air bubbles will collect in the bottle during coolant circulation
Degas uses a Radiator Cap with a 14-18lbs rating so not a "sealed" bottle, it has an overflow hose but it just exits to under the vehicle in case Degas bottle/Radiator was over filled, extra coolant and any air is pushed out when coolant is heated and expands
As engine cools back down, air is sucked back in thru the smaller valve in "rad cap"
Systems with Degas bottle often have TWO rad caps, the one on the radiator IS a SEALED cap, not a pressure cap, the one on the Degas bottle is a pressure cap, so not sealed
Newer radiators for Degas system may not even have a rad cap opening
Overflow system is different in that Radiator has the pressure cap and radiator should be filled to the top, no air at all
When coolant is heated it expands and when pressure reaches cap rating, 14-18psi, hot coolant, and any air, is sent through the Over Flow hose to the BOTTOM of the over flow tank
Bottom of the tank is important, it means that any air that comes out of radiator will bubble up to the top
Overflow tank is not sealed just has a vented cap
When engine cools back down the smaller valve in rad cap sucks coolant back in from the BOTTOM of over flow tank
So radiator stays Topped Up after each cool down
Overflow tank setup allows you to add coolant while engine is still warm IF...........coolant level in the tank is lower than the WARM line when engine is fully warmed up
If overflow tank is bubbling that's because radiator has AIR in it and its being push out, its not "boiling hot" in overflow tank, but could be in engine, lol
Overflowing overflow tank usually means you have a head gasket or cracked head leaking pressure into cooling system, this pushes out coolant and replaces it with air, causing over heating, and then bubbles in the tank
Overflowing Degas system means the same thing
One is not better than the other as far as cooling systems
Degas became more popular again as radiators got LOWER in the front so couldn't be the High Point
Not Ranger specific since they still had higher front ends unlike cars, but in the case of the 2.3l Duratec which was made for cars Ford just didn't want to redesign the cooling system so stayed with Degas
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