Hey all,
First time using a forum for information, I have a 2001 Ranger 3.0 with 125k miles that blew the heater to engine hose yesterday. I was able to limp it home to replace the hose and fill coolant. Now the truck overflows the coolant at idle but doesn't overheat. What should I look at as the problem? There's no water in the oil no engine light or rough idle. Could this be a stuck thermostat?

 

Welcome to the forum

Could be Rad cap is not holding coolant in the engine/radiator
There are two types of cooling systems, the "Overflow system" and the "Degas" system
You have an overflow system
All fluids EXPAND in volume when heated, so coolant expands when heated, say a gallon of coolant becomes 1.5gallons when hot, this expansion causes pressure to build up in the engine and radiator, this pressure is good as it raises the boiling point of the coolant(how a pressure cooker works) to prevent "boil over"

Radiator cap has a larger spring that holds it closed until this expansion of coolant reaches 15psi pressure in the radiator then it will be pushed up and let a little hot coolant flow over to the Overflow tank until pressure is just under 15psi
If rad caps spring fails then coolant is free to expand into the overflow bottle so no pressure builds up in the engine/radiator, which could overflow the tank

Radiator cap also has a smaller valve in the center, after engine is shut off the coolant cools down, and SHRINKS in volume, if any coolant was sent over to the overflow tank during driving then pressure in the engine/radiator will drop to -1psi and this small valve is sucked open pulling coolant back in from the overflow tank, this is why the hose is at the BOTTOM of the overflow tank

Stuck Closed thermostat would cause overheating
Stuck Open thermostat would cause delayed or no full warm up, lower than normal temp gauge reading


Blown head gasket
This is caused from overheating any engine, even once, the head metal expands too much and crushes the head gasket between head and block
Bubbles in the coolant are a common sign of blown head gasket
Until there are enough bubbles there is no overheating, the water pump circulates the coolant fast enough to keep the air from building up in the head(high point)
But at some point enough air is being pumped in by the cylinder with the blown gasket that the air stays in the head(air dam and blocks coolant flow) and engine over heats

 

Burp the system. There is air in there. Pull the rad cap when cold, fill with coolant than turn the truck on.
Pinch the lower rad hose a few time to get any trap air & do the same for the upper hose.