Hello All I'm new to the Ranger group and thanks for allowing me to join. I have a question how commen is it for your overfill coolant tank to be 3/4 full after driving for 5mins?

 

Welcome to the forum

I assume it was at the FULL line when cold
Then not very common

It would indicate a failing radiator cap, best case
Worst case a cracked head or blown head gasket

Cooling systems are setup to maintain 14-16psi pressure inside after engine warms up, this is done by the radiator cap's main spring
The pressure comes from the fact that all liquids expand in volume as they are heated up, so a gallon of coolant at 60degF becomes 1.5gal when heated to 180degF, this creates the pressure inside the engine and radiator, if it exceeds "cap rating" then some of the hot coolant is released and flows over to the over flow tank.
If rad cap spring was bad then coolant would flow over to over flow tank within a few minutes of run time as it heated up
The pressure is needed inside the engine/radiator to raise the boiling point of the coolant from 230degF to 265degF, just FYI

If the 4.0l OHV engines(1990-2000) are ever overheated, even once, then they tend to get a cracked head between valve seats, this allows "air" from inside a cylinder to be pumped in to the cooling system, raising the pressure above the 14-16psi cap rating and forces coolant into the over flow tank much sooner than "normal"
It would also be followed by bubbles in the over flow tank, the "air" being pumped in would find its way to top of radiator and then be pushed out to over flow tank and bubble up, its not "boiling" coolant just the bubbles from "air" in cylinders coming it
And it would lead to overheating, as the "air" builds up in the heads it doesn't cool as well as coolant does

Overheating ANY engine can cause head gasket to be CRUSHED, head metal expands too much and crushes the head gaskets metal ring for a cylinder, same symptom as above, "air" from that cylinder is pumped into cooling system which cause high pressure in the cooling system

 

Thanks for the awesome information I just replaced the rad cap and will see what happens in the next day or so.

 

Hello Again,

Prior to me posting on the forum about this issue I was having. I had just put the new rad cap, today after a drive coolant temp came down to normal.
I did notice the drain on the rad put plugged with build up took 10 minutes to get it to flow. So I've ordered a new Rad.

Thanks again for the great Info.